Worked example: analyzing an ocean food web

WEBVTT
Kind: captions
Language: en

00:00:00.350 --> 00:00:01.950
- [Instructor] So this
diagram right over here

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describes a food web.

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And a food web models

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how energy and matter
moves in an ecosystem.

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And we're going to use this food web

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to answer some questions,

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to make sure we understand food webs.

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So the first thing I'm
going to ask you is,

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what are the producers in this food web.

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Pause this video and see
if you can figure this out.

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All right, now let's work
through this together.

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The producers are the ones

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that can take matter in the
ecosystem, in the environment,

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and energy from an outside
source, usually the sun,

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in order to construct themselves

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and also in order to store that energy.

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And when you look at a food web,

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producers are going to have
arrows pointed away from them.

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So they're getting their energy

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and their matter from the environment

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and then other people are
going to consume them.

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And we're gonna talk about
them in a little bit.

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But we can see here that
the kelp is a producer.

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This is an aquatic ecosystem
that we're looking at.

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But the kelp is a producer.

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There's no arrows going into it

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so it's using the sun and
elements and atoms and molecules

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that it's finding in its
environment in order to grow.

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And we can also see that's the
case with the phytoplankton.

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It's also able to do
photosynthesis, like the kelp,

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and capture that energy from the sun.

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Now, the next question I have
is, where are the decomposers?

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Pause the video and
try to answer that one.

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All right.

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Now you might remember decomposers,

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you can think about that's where a lot

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of the energy and the matter ends up,

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to be decomposed and then recycled again

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and be reused again by the producers.

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And if you're looking at a food web,

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these would-be organisms

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where all of the arrows point to them,

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but you don't have any
arrows coming from them.

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And over here, it's
clear that the bacteria,

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and there's many different
species of bacteria

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or many different types of bacteria,

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but the bacteria here are the decomposers.

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Next, I am going to ask you

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about the primary consumers.

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Pause this video and see
if you can identify them.

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Well, primary consumers are consumers

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that directly eat the producers.

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And so let's see, who is eating the kelp?

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You have have the sea
urchin eating the kelp.

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You have the fish eating the kelp.

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Who's eating the phytoplankton?

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You have the zooplankton
eating the phytoplankton.

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So these are primary consumers.

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Next, let's think about
secondary consumers.

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Secondary.

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These are those that
eat primary consumers.

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Pause the video and think about that.

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Well, let's think about

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who's eating these primary consumers.

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You have the otter.

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What's interesting is the
fish is a primary consumer

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because it consumes kelp,
which is a producer,

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but it also can consume
another primary consumer.

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So, a fish, or the fish here,

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are both primary and secondary consumers.

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And then I think we are done.

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So you can see the general notions here.

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The producers have arrows
going away from them

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but no arrows coming into them

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because they're getting
their energy from the sun

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and they're using the
matter in the ecosystem.

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Primary consumers are
eating those producers,

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and you can see they have
arrows going into them.

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And then they're eaten by other
folks, secondary consumers,

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or those that eat the primary consumers.

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And then last but not least,
you have the bacteria,

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which are decomposers, where
the arrows all point to them

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and they continue to use that energy

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and then they take those
nutrients, those molecules,

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and they put 'em back into the ecosystem

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so that it can be recycled again.


Matter and energy in food webs

WEBVTT
Kind: captions
Language: en

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- [Narrator] In this video,

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we're gonna talk about food webs,

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which is really just a way

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of picturing how all of the matter,

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and how all of the energy
flows inside of an ecosystem.

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Now, when I talk about matter,

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I'm talking about the
atoms in an ecosystem,

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the molecules, when you look at your hand,

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it is made up of atoms.

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And it turns out that the matter

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is not created or destroyed,

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it's just recycled
throughout an ecosystem.

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And we're going to see that in a second.

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And then when we talk about energy,

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it's the energy that your
cells need to be alive,

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the energy you need to be
alive, to not just exist,

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but to do things, to think, to move.

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And so this food web
essentially describes that.

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As we've talked about in other videos,

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in most ecosystems, the
great majority of the energy

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in an ecosystem comes from the sun.

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So what we have here is
the sun produces energy.

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It travels to Earth,

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and then you have organisms,
which we would call producers,

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that are able to take that
light energy from the sun,

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and then take atoms and
molecules from its environment,

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things like carbon dioxide in the air,

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things like water, and other nutrients.

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And it's able to construct itself

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using that energy from the sun.

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Now, when it constructs itself,

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it not only gives it structure,

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but it's also able to store energy.

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And right over here, we have
several producers depicted.

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We have this tree here, which
is able to do photosynthesis.

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We have the grass here that's
able to do photosynthesis,

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and it's not just plants.

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You have things like algae,

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and other microorganisms that
are able to be producers.

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But then we have things like this bunny,

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and this bunny is not
able to harness energy

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from the sun by itself.

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In order for it to get
its energy and its matter,

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it needs to eat one of these producers,

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probably some of this grass.

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And so we would call this
bunny right over here,

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this rabbit, we would call it a consumer.

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And it is a consumer,

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you could think of it as
both matter and energy.

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When it eats that plant,

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those atoms are then able
to make up the bunny.

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It will poop out a little bit,

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so some of the matter might
end up right over here.

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But then also there is energy

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that is stored in those molecules,

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and that rabbit can use that
energy to exist and live,

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but as it does it,

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it does release some of the
energy in the form of heat.

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And actually, even
producers need to use energy

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in order to live.

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And as they do that,
they also release heat.

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Now we have this fox, the
fox is not a vegetarian.

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It does not eat, it does not eat trees.

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It likes to eat things
like squirrels and bunnies,

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but big picture, it's not
producing its own food.

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It's consuming food, so
it also is a consumer.

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We can differentiate more in the future

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between things that eat plants,

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and things that eat other animals.

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And you can see in this food web,

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we draw an arrow from the
thing that is being consumed

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to the thing that is doing the consuming.

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So a rabbit consumes a plant,

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and so the arrow goes from
the plant to the rabbit.

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A fox consumes a rabbit or a squirrel,

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so an arrow goes from the rabbit
or the squirrel to the fox.

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Now some of you might have noticed

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that we have these arrows
that are pointing downward.

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And so let me scroll down a little bit.

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And we see these
microorganisms, the worms,

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the fungi, the mushrooms right over here,

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and we call these decomposers.

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Now what decomposers are really doing

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is what what you would imagine,

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breaking down all of the
things that might die,

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the poop that is falling down.

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And by doing that, it's
continuing to recycle that matter.

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And when it decomposes,

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those atoms are released
back into the soil,

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or the atmosphere,

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and then that can be
reused by the producers.

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And once again, every
organism in this food web

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is using some of the
energy in order to exist,

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and do whatever it needs to do.

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And some of that energy is being released

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as heat in every single situation.


Photosynthesis in ecosystems

WEBVTT
Kind: captions
Language: en

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- [Instructor] When you look at

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a rainforest ecosystem like this,

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one of the obvious questions may be,

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where do these plants come from?

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How do they grow?

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They're growing all the time,

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they're getting larger
and larger and larger.

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Where does that mass, where
does that matter come from?

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Pause this video and think about that.

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Well, you might already be
guessing where it comes from.

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It might have something
to do with photosynthesis,

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which we go into a lot
of depth in other videos,

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but this is a process where
you have carbon dioxide

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from the air in conjunction with water

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that primarily is coming from the soil,

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and it uses energy from the sun

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so I'll just draw that in,

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these yellow squiggles
coming from the sun,

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in order to do two things.

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The plant is going to be building itself

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using the matter in the
carbon dioxide and the water

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while also expelling
oxygen as a byproduct.

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And this matter that the plant is able

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to take from its environment
with photosynthesis

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is used to both become
the structure of the plant

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and a store of energy
in the form of sugars.

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Now, animals like you and me,

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we get our energy by
then eating these plants.

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And how do we unlock that energy?

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Well, that's where this
oxygen is really useful.

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That's why we need to breathe oxygen

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because by breathing the oxygen,

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we can essentially do
photosynthesis in reverse,

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and we can break down this matter,

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these sugars, that we're
getting from plants.

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So all of this biomass is coming

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from essentially water from the soil

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and carbon dioxide,
and energy from the sun

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is used essentially put it together.

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There might be a few other nutrients

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that are also coming from the
environment, like the soil,

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but primarily the water
and the carbon dioxide.


Analog vs. digital signals

WEBVTT
Kind: captions
Language: en

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- [Narrator] In this
video we're gonna think

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about analog versus digital signals.

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And one way to think about the difference

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is an analog signal is trying to reproduce

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exactly in some type of a signal

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what is going on while a
digital signal is converting it,

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usually the ones and zeros which
then can be converted back.

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So the first thing to realize

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is that almost any type of information,

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it might be how intense a certain color is

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or how bright it is or it
might be a frequency of sound,

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all of that can be represented as numbers.

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So whether you're trying to transmit

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an analog signal or a digital signal

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it really is about how do
you communicate numbers

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using some type of signal.

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And as an example, let's
say that you are trying

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to communicate the number 24 to someone.

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Well, one way to do it
is as an analog signal.

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You could have, as time goes on,

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maybe this is some type
of voltage across a wire.

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And if you put that voltage

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right at what the other person

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receiving it could interpret it as 24,

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well then you would
transmit it and they might,

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if they get this nice clean
signal, see it as a 24.

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Now the problem with analog signals

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is what happens when
you have interference.

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So let's say this is a really long wire

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that you're transmitting it over

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or you're transmitting
it using radio waves,

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and there's a lot of bad weather

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or other types of interference.

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And so on the receiving end
even though you transmit

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something that looks like
this, that is a clear 24,

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the other person might
get something like this.

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And so this isn't obvious
that is a 24 anymore.

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It goes between 20 and 30,

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and this could be interpreted as static

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or a garbled up message

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or just doesn't sound as clean

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depending on what the 24
is trying to represent,

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or the image isn't as clear,

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if this is representing say the lightness

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or the brightness of a pixel.

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Now the other option is to
convert that 24 into binary.

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So 24 can be represented
in binary as 1 1 0 0 0.

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Now we have other videos on Khan Academy

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that explain how to convert back and forth

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between our decimal
system that has 10 digits,

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zero through nine, and the binary system,

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which has two digits, zero and one.

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But in case you're curious,

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and you don't have to understand this

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to know the difference between
analog and digital signals

00:02:30.910 --> 00:02:32.410
and when they might be useful,

00:02:32.410 --> 00:02:35.150
this first place is the ones
place just as you're used to.

00:02:35.150 --> 00:02:37.020
But instead of this being the tens place,

00:02:37.020 --> 00:02:38.560
this is the twos place.

00:02:38.560 --> 00:02:40.150
Instead of this being the hundreds place,

00:02:40.150 --> 00:02:41.910
this is the forth place.

00:02:41.910 --> 00:02:44.530
Instead of this being the thousands place,

00:02:44.530 --> 00:02:46.300
this is the eights place.

00:02:46.300 --> 00:02:49.540
And instead of that being
the ten thousandths place,

00:02:49.540 --> 00:02:51.550
that's the sixteens place.

00:02:51.550 --> 00:02:52.750
So one way to think about it,

00:02:52.750 --> 00:02:57.360
1 1 0 0 0 means 1/16 and 1/8.

00:02:57.360 --> 00:03:00.240
And if you add those
together, you would get 24.

00:03:00.240 --> 00:03:02.633
What would that actually
look like as a signal?

00:03:03.480 --> 00:03:05.640
Well, it could look something like this.

00:03:05.640 --> 00:03:07.450
Where the person interpreting it knows

00:03:07.450 --> 00:03:09.220
that over this first time period

00:03:09.220 --> 00:03:10.560
whatever voltage you're getting,

00:03:10.560 --> 00:03:12.350
that tells you your first digit,

00:03:12.350 --> 00:03:13.980
that over the next time, period

00:03:13.980 --> 00:03:16.420
whatever voltage you're getting
tells you the second digit,

00:03:16.420 --> 00:03:18.090
and so on and so forth.

00:03:18.090 --> 00:03:22.540
So this would be 1 1 0 0 0.

00:03:22.540 --> 00:03:24.130
Now, why is this useful?

00:03:24.130 --> 00:03:25.620
Well, let's think about the situation

00:03:25.620 --> 00:03:28.750
where all of a sudden
there's interference again.

00:03:28.750 --> 00:03:30.040
Now with the interference,

00:03:30.040 --> 00:03:35.040
you could still interpret
this exactly as 1 1 0 0 0.

00:03:35.190 --> 00:03:37.080
Remember the person receiving the signal

00:03:37.080 --> 00:03:38.760
or the system receiving the signal

00:03:38.760 --> 00:03:39.850
is just trying to determine

00:03:39.850 --> 00:03:41.530
whether it's getting a one or a zero.

00:03:41.530 --> 00:03:44.800
So even though this is being
perturbed right over here,

00:03:44.800 --> 00:03:46.750
it's clear that we are at a one.

00:03:46.750 --> 00:03:49.750
We know that it would not be a 0.9 or 1.1.

00:03:49.750 --> 00:03:52.040
It has to be either one or zero.

00:03:52.040 --> 00:03:55.890
So it's clear that over
this part right over here,

00:03:55.890 --> 00:03:57.100
you're getting a one,

00:03:57.100 --> 00:03:59.550
and then over here you're
getting three zero.

00:03:59.550 --> 00:04:03.620
So they would be able to
interpret it as 1 1 0 0 0,

00:04:03.620 --> 00:04:06.153
which is once again exactly 24.


Absorption and reflection

WEBVTT
Kind: captions
Language: en

00:00:00.270 --> 00:00:02.060
- [Instructor] I'm showing
you this beautiful picture

00:00:02.060 --> 00:00:05.490
of snow-capped mountains
overlooking this Alpine Lake,

00:00:05.490 --> 00:00:08.360
because there's a lot
of light moving around.

00:00:08.360 --> 00:00:10.130
Now, we're gonna talk
about the different ways

00:00:10.130 --> 00:00:13.890
that light can interact
with different media,

00:00:13.890 --> 00:00:17.480
but what I'm talking about can
generally be true of waves.

00:00:17.480 --> 00:00:19.160
But focusing on light for a second,

00:00:19.160 --> 00:00:20.420
we've already talked about how,

00:00:20.420 --> 00:00:22.130
if we go from one medium to another,

00:00:22.130 --> 00:00:24.670
if we're transmitting from
one medium to another,

00:00:24.670 --> 00:00:27.590
that the light can go through a medium.

00:00:27.590 --> 00:00:30.580
And as it goes to that boundary
from one medium to another,

00:00:30.580 --> 00:00:32.140
its direction can change.

00:00:32.140 --> 00:00:34.060
And we talk about that in another video,

00:00:34.060 --> 00:00:36.020
and we call that refraction.

00:00:36.020 --> 00:00:38.090
We have refraction when
we enter the material,

00:00:38.090 --> 00:00:40.790
and then we have more
refraction when we get out.

00:00:40.790 --> 00:00:43.700
Now, there's other things
that light can also do,

00:00:43.700 --> 00:00:46.060
and that's the focus of this video,

00:00:46.060 --> 00:00:48.570
reflection and absorption.

00:00:48.570 --> 00:00:49.800
Now you probably have a sense

00:00:49.800 --> 00:00:51.550
of what happens with reflection.

00:00:51.550 --> 00:00:54.080
We can see a reflection of the mountains

00:00:54.080 --> 00:00:55.600
in the lake right over here.

00:00:55.600 --> 00:00:57.990
And the reason why we
can see the reflection

00:00:57.990 --> 00:00:59.540
of the mountains and the lake here

00:00:59.540 --> 00:01:03.070
is because light that is
coming from this mountain

00:01:03.070 --> 00:01:07.670
is hitting the lake and then
it is bouncing off of it,

00:01:07.670 --> 00:01:11.830
and then coming to an observer's
eyeball right over here,

00:01:11.830 --> 00:01:15.180
and so they see the light
that's coming from here,

00:01:15.180 --> 00:01:17.180
they see it as coming from over here,

00:01:17.180 --> 00:01:18.520
because it is bounced off.

00:01:18.520 --> 00:01:22.160
And so reflection is exactly
that, light is coming in,

00:01:22.160 --> 00:01:24.210
it hits that other material,

00:01:24.210 --> 00:01:27.300
and then if it bounces
off, that's reflection.

00:01:27.300 --> 00:01:30.320
Now, the other thing that
light can do is get absorbed.

00:01:30.320 --> 00:01:33.610
Absorbed is when the
material doesn't reflect any,

00:01:33.610 --> 00:01:35.340
it doesn't transmit any through it,

00:01:35.340 --> 00:01:37.020
if it's completely absorbing.

00:01:37.020 --> 00:01:40.070
And so that would be
a situation like this.

00:01:40.070 --> 00:01:41.740
Now, the reality in the real world

00:01:41.740 --> 00:01:43.830
is we have oftentimes a little bit

00:01:43.830 --> 00:01:46.170
of all of the above happening.

00:01:46.170 --> 00:01:50.800
For example, when we look
at this white snow up here,

00:01:50.800 --> 00:01:52.730
this is reflecting pretty well.

00:01:52.730 --> 00:01:55.150
So there's light that's
coming from the sun,

00:01:55.150 --> 00:01:57.420
or maybe it's being
reflected off of the clouds,

00:01:57.420 --> 00:01:59.030
or maybe it's getting through the clouds,

00:01:59.030 --> 00:02:02.320
and when it hits that
snow, it gets reflected.

00:02:02.320 --> 00:02:03.950
Now, the reason why we
don't see a reflection

00:02:03.950 --> 00:02:06.370
the way that we see in
this lake right over here

00:02:06.370 --> 00:02:09.370
is the snow reflects it in
all different directions,

00:02:09.370 --> 00:02:11.310
but the fact that it's
that bright color, in fact,

00:02:11.310 --> 00:02:13.260
you might need sunglasses
to look at the snow,

00:02:13.260 --> 00:02:15.270
just as much as you
need to look at the sky,

00:02:15.270 --> 00:02:18.000
is that it's reflecting most of the light.

00:02:18.000 --> 00:02:20.690
But if you go down here
where we see the trees,

00:02:20.690 --> 00:02:23.070
the same light from the sky is hitting it,

00:02:23.070 --> 00:02:25.840
but not as much light is
coming back to our eye,

00:02:25.840 --> 00:02:28.530
and that's because this
part of the mountain,

00:02:28.530 --> 00:02:31.260
it might be trees, it might
be rock, it might be dirt,

00:02:31.260 --> 00:02:34.040
is absorbing more of the light,

00:02:34.040 --> 00:02:35.330
but it's still reflecting some.

00:02:35.330 --> 00:02:36.890
We can still see it a little bit.

00:02:36.890 --> 00:02:39.860
So this might be where most
of it is getting absorbed,

00:02:39.860 --> 00:02:42.610
but a little bit of the
light gets reflected,

00:02:42.610 --> 00:02:44.670
which we can see right over there.

00:02:44.670 --> 00:02:46.970
And if we think about the
water right over here,

00:02:46.970 --> 00:02:49.160
some of the light is
probably making it through

00:02:49.160 --> 00:02:51.720
and probably refracting as it does so.

00:02:51.720 --> 00:02:53.570
Some of it is getting absorbed

00:02:53.570 --> 00:02:55.210
as it makes its way through the water.

00:02:55.210 --> 00:02:56.470
If this was a really deep lake,

00:02:56.470 --> 00:02:58.770
when you get to the bottom,
it could be very, very dark.

00:02:58.770 --> 00:03:00.180
And then, as we talked about,

00:03:00.180 --> 00:03:02.000
it looks like a good bit is reflecting.

00:03:02.000 --> 00:03:03.900
When we look at the image
that we see in the lake,

00:03:03.900 --> 00:03:07.290
it looks almost as
bright as the real thing.

00:03:07.290 --> 00:03:10.830
So as I said, this is
happening all around you.

00:03:10.830 --> 00:03:13.380
In fact, when people make
fancy computer graphics,

00:03:13.380 --> 00:03:16.190
they actually try to do
exactly what the light would do

00:03:16.190 --> 00:03:19.590
in the real world to make
an image for your eyes

00:03:19.590 --> 00:03:21.490
that look like the real world.

00:03:21.490 --> 00:03:23.150
And they're thinking exactly about this.

00:03:23.150 --> 00:03:24.960
How much gets through the substance?

00:03:24.960 --> 00:03:27.700
How much gets reflected,
and how much gets absorbed?

00:03:27.700 --> 00:03:28.820
And as I mentioned,

00:03:28.820 --> 00:03:31.030
it's not just light waves that do this.

00:03:31.030 --> 00:03:33.923
Waves in general can do all of the above.


Wave transmission

WEBVTT
Kind: captions
Language: en

00:00:00.450 --> 00:00:02.050
- [Instructor] When we're
talking about waves,

00:00:02.050 --> 00:00:06.650
transmission is when a
wave passes from a material

00:00:06.650 --> 00:00:08.300
into another one.

00:00:08.300 --> 00:00:11.170
For example, here we have the sun,

00:00:11.170 --> 00:00:14.580
93 million miles away on average,

00:00:14.580 --> 00:00:17.390
and imagine the different materials

00:00:17.390 --> 00:00:20.260
that the light has to
travel through from the sun

00:00:20.260 --> 00:00:24.320
to, say, hit one of these sand
particles right over here.

00:00:24.320 --> 00:00:27.410
Think about what it needs
to be transmitted through.

00:00:27.410 --> 00:00:30.780
What's going to travel
through 93 million miles

00:00:30.780 --> 00:00:32.130
of the vacuum of space,

00:00:32.130 --> 00:00:34.270
and that's one of the amazing
things about light waves

00:00:34.270 --> 00:00:35.430
is that they don't need a medium.

00:00:35.430 --> 00:00:38.660
They can travel through
vacuum, through emptiness,

00:00:38.660 --> 00:00:41.510
but then it's gonna travel
through several miles

00:00:41.510 --> 00:00:43.230
of Earth's atmosphere.

00:00:43.230 --> 00:00:44.800
So it's going to travel
through several miles

00:00:44.800 --> 00:00:45.633
of Earth's atmosphere.

00:00:45.633 --> 00:00:48.321
It will hit these, the
lenses of these sunglasses,

00:00:48.321 --> 00:00:50.880
the light will travel through
the lenses of the sunglass,

00:00:50.880 --> 00:00:54.280
little sunglass has some
width or some depth to it,

00:00:54.280 --> 00:00:56.570
and then it'll go out onto the other side

00:00:56.570 --> 00:00:59.790
and it will hit the sand right over here.

00:00:59.790 --> 00:01:01.340
Now, one thing you might realize

00:01:01.340 --> 00:01:05.600
is the amount of transmission
and what gets transmitted

00:01:05.600 --> 00:01:08.270
is dependent on the
wavelengths of the wave,

00:01:08.270 --> 00:01:10.030
in this case, the wavelengths of light,

00:01:10.030 --> 00:01:13.630
and also about the material
that they are going through.

00:01:13.630 --> 00:01:16.240
So for example, these
sunglasses right over here,

00:01:16.240 --> 00:01:20.100
many sunglasses try to keep out
UV light, ultraviolet light,

00:01:20.100 --> 00:01:22.150
which is a higher frequency
than visible light,

00:01:22.150 --> 00:01:24.010
but that's what causes sunburns

00:01:24.010 --> 00:01:26.440
and that can also damage your eyes.

00:01:26.440 --> 00:01:29.670
So those high frequencies
are not making it through.

00:01:29.670 --> 00:01:32.990
And we can also see that this
sunglass right over here,

00:01:32.990 --> 00:01:34.850
it kind of has an orange color,

00:01:34.850 --> 00:01:36.940
which means that things that are closer

00:01:36.940 --> 00:01:38.130
to that end of the spectrum,

00:01:38.130 --> 00:01:40.390
closer to the red, the
oranges, and the yellows

00:01:40.390 --> 00:01:41.310
are getting through,

00:01:41.310 --> 00:01:44.030
which means that it's
filtering out blue light.

00:01:44.030 --> 00:01:47.350
So the blue light, isn't getting
transmitted through as much

00:01:47.350 --> 00:01:49.370
as say the red, orange, and yellow light

00:01:49.370 --> 00:01:52.370
and that's why we see this
as red, orange, or yellow.

00:01:52.370 --> 00:01:53.203
And then, of course,

00:01:53.203 --> 00:01:55.390
the light will get to that sand particle.

00:01:55.390 --> 00:01:57.010
Now transmission, as I mentioned,

00:01:57.010 --> 00:01:59.310
it isn't just about light waves.

00:01:59.310 --> 00:02:00.280
We could talk about

00:02:00.280 --> 00:02:02.010
one of our other favorite types of waves,

00:02:02.010 --> 00:02:04.060
for example, sound waves.

00:02:04.060 --> 00:02:05.880
If you are in a room,

00:02:05.880 --> 00:02:08.210
you have probably experienced the fact

00:02:08.210 --> 00:02:10.930
that even if you were to close the door,

00:02:10.930 --> 00:02:13.230
and I do this a lot because
I record a lot of videos.

00:02:13.230 --> 00:02:16.320
This is me in my little
closet recording a video.

00:02:16.320 --> 00:02:19.040
This is a top view for
what I'm doing right now.

00:02:19.040 --> 00:02:21.840
A lot of times my kids are
in other parts of the house

00:02:21.840 --> 00:02:24.650
and they're making a lot of noise.

00:02:24.650 --> 00:02:26.020
And as we've talked about,

00:02:26.020 --> 00:02:28.920
sound waves are nothing but
traveling pressure waves

00:02:28.920 --> 00:02:29.753
through the air.

00:02:29.753 --> 00:02:32.200
Those air particles are
knocking one into another.

00:02:32.200 --> 00:02:33.840
But in order to make it to me,

00:02:33.840 --> 00:02:35.460
they need to get through that wall,

00:02:35.460 --> 00:02:36.430
and the way they do that

00:02:36.430 --> 00:02:39.280
is they get transmitted through that wall.

00:02:39.280 --> 00:02:41.800
So those air particles make the particles

00:02:41.800 --> 00:02:43.420
or make the atoms or the molecules

00:02:43.420 --> 00:02:46.450
in the walls start vibrating,
they bump into each other,

00:02:46.450 --> 00:02:49.720
and then the particles on
the other side of the wall

00:02:49.720 --> 00:02:52.400
will bump into the air
in my little closet,

00:02:52.400 --> 00:02:55.000
and then we will have, once again,

00:02:55.000 --> 00:02:57.020
the sound waves make it to me.

00:02:57.020 --> 00:02:59.510
Now, the overall magnitude of sound,

00:02:59.510 --> 00:03:02.050
the volume of the sound,
will likely be diminished

00:03:02.050 --> 00:03:04.440
and not all of the
frequencies of the sound

00:03:04.440 --> 00:03:06.030
will be transmitted equally.

00:03:06.030 --> 00:03:08.140
Different frequencies of sound waves

00:03:08.140 --> 00:03:11.000
are better at traveling
through certain materials,

00:03:11.000 --> 00:03:14.010
just as we talked about with light waves.

00:03:14.010 --> 00:03:15.930
So now when you look at
the world around you,

00:03:15.930 --> 00:03:17.480
or you hear the world around you,

00:03:17.480 --> 00:03:19.270
or look at other types of waves,

00:03:19.270 --> 00:03:21.210
just think about how it's transmitting

00:03:21.210 --> 00:03:23.723
from one material into another.


Refraction in a glass of water

WEBVTT
Kind: captions
Language: en

00:00:00.290 --> 00:00:01.800
- [Instructor] So
something very interesting

00:00:01.800 --> 00:00:04.050
is clearly going on when
we look at this pencil

00:00:04.050 --> 00:00:06.000
dipped in this cup of water.

00:00:06.000 --> 00:00:10.390
We would expect if maybe there
was no water in this glass

00:00:10.390 --> 00:00:13.250
that we would just see the
pencil continue straight down

00:00:13.250 --> 00:00:16.740
in a line that looks something like that.

00:00:16.740 --> 00:00:19.240
But that's clearly not what we are seeing.

00:00:19.240 --> 00:00:21.340
It looks like, once we fill it with water,

00:00:21.340 --> 00:00:22.830
at least to our eye,

00:00:22.830 --> 00:00:25.760
it looks like the pencil
gets bent or broken

00:00:25.760 --> 00:00:27.500
or bent in some way.

00:00:27.500 --> 00:00:28.850
And this notion,

00:00:28.850 --> 00:00:31.790
you might have heard
people call it refraction.

00:00:31.790 --> 00:00:34.490
But it's interesting
thing about exactly why

00:00:34.490 --> 00:00:35.780
this is happening.

00:00:35.780 --> 00:00:37.580
And I'll give you a hint.

00:00:37.580 --> 00:00:40.170
This is all about the bending of light.

00:00:40.170 --> 00:00:42.700
And it's not just light that can get bent

00:00:42.700 --> 00:00:45.490
as it goes from one medium into another.

00:00:45.490 --> 00:00:48.463
It can be any kind of wave.

00:00:48.463 --> 00:00:51.100
So let's think about
what's going on over here.

00:00:51.100 --> 00:00:52.760
So first, let's think about
the part of the pencil

00:00:52.760 --> 00:00:54.370
that is above the water.

00:00:54.370 --> 00:00:56.360
So this part right over here,

00:00:56.360 --> 00:00:59.710
the light is actually
reflecting off of this pencil,

00:00:59.710 --> 00:01:02.120
and then it's bouncing
straight into our eye.

00:01:02.120 --> 00:01:03.970
So just imagine a path from this dot

00:01:03.970 --> 00:01:05.250
straight into your eye.

00:01:05.250 --> 00:01:07.980
Once again, from here, it's
going straight into your eye.

00:01:07.980 --> 00:01:11.360
When we go over here, it still
doesn't look too distorted.

00:01:11.360 --> 00:01:13.750
So you have light that's
going straight to your eye.

00:01:13.750 --> 00:01:15.614
It's going through the side of the glass

00:01:15.614 --> 00:01:18.553
and then getting to your eye.

00:01:18.553 --> 00:01:20.120
But then once we get under the water,

00:01:20.120 --> 00:01:21.360
something's interesting.

00:01:21.360 --> 00:01:25.430
You would expect the point
that would have been here

00:01:25.430 --> 00:01:27.340
would then go straight to your eye

00:01:27.340 --> 00:01:28.980
just like everything up here.

00:01:28.980 --> 00:01:30.730
But it turns out that that light,

00:01:30.730 --> 00:01:35.020
once it transitions from going
from the water to the glass

00:01:35.020 --> 00:01:37.300
and then the air, it bends.

00:01:37.300 --> 00:01:40.630
So at the interface between the media,

00:01:40.630 --> 00:01:42.080
between those different materials

00:01:42.080 --> 00:01:43.620
that the light is traveling through,

00:01:43.620 --> 00:01:44.935
instead of going towards your eye,

00:01:44.935 --> 00:01:48.470
it gets bent, in this case to our left.

00:01:48.470 --> 00:01:50.850
And so that's why when
we look straight on here,

00:01:50.850 --> 00:01:53.999
we don't see anything in
this region right over here.

00:01:53.999 --> 00:01:56.516
But the light that was going

00:01:56.516 --> 00:02:00.610
from the pencil towards
this part of the glass,

00:02:00.610 --> 00:02:02.250
which typically you would not see,

00:02:02.250 --> 00:02:04.070
that would have typically
just gone straight

00:02:04.070 --> 00:02:06.250
in that direction and not hit your eye,

00:02:06.250 --> 00:02:08.600
well, now that is getting refracted.

00:02:08.600 --> 00:02:10.330
It's getting bent to the left

00:02:10.330 --> 00:02:12.860
so that now that light hits your eye.

00:02:12.860 --> 00:02:15.990
So that's why you see what
looks like a broken pencil.

00:02:15.990 --> 00:02:19.120
It's all about the light getting bent

00:02:19.120 --> 00:02:22.550
as it exits the water
and goes into the glass

00:02:22.550 --> 00:02:23.923
and then the air.


Mechanical waves and light

WEBVTT
Kind: captions
Language: en

00:00:00.550 --> 00:00:02.870
- [Instructor] Let's talk about waves.

00:00:02.870 --> 00:00:05.360
So let's imagine that
you were to take a string

00:00:05.360 --> 00:00:08.150
and attach it at one end to a wall,

00:00:08.150 --> 00:00:09.390
and then on the other end,

00:00:09.390 --> 00:00:11.700
you were to wiggle it up and down.

00:00:11.700 --> 00:00:14.280
Well, then you would have made a wave.

00:00:14.280 --> 00:00:17.690
You would see a pattern
that looks like this.

00:00:17.690 --> 00:00:21.010
Now, what would be a good
definition for a wave?

00:00:21.010 --> 00:00:24.610
Well, we could call it
a traveling disturbance.

00:00:24.610 --> 00:00:26.130
Well, what does that mean?

00:00:26.130 --> 00:00:27.750
Well, we're disturbing the rope.

00:00:27.750 --> 00:00:30.180
If we didn't move it, if
we just held it straight,

00:00:30.180 --> 00:00:32.360
it might look something like that,

00:00:32.360 --> 00:00:34.350
or it might just hang down a little bit,

00:00:34.350 --> 00:00:37.700
but clearly we are now
moving it up and down,

00:00:37.700 --> 00:00:40.740
and those movements are
disturbing that rope

00:00:40.740 --> 00:00:44.190
and that disturbance can
move along that rope.

00:00:44.190 --> 00:00:46.200
Now, we see waves not just in ropes

00:00:46.200 --> 00:00:48.020
that are moving up and down.

00:00:48.020 --> 00:00:50.590
You have probably seen water waves.

00:00:50.590 --> 00:00:52.740
If you were to take a
tank of water like this

00:00:52.740 --> 00:00:55.110
and if you were to start
pressing on one end

00:00:55.110 --> 00:00:56.400
of the water here,

00:00:56.400 --> 00:00:59.760
you would see these wave forms that start.

00:00:59.760 --> 00:01:03.600
We can also see that with
sound and sound waves.

00:01:03.600 --> 00:01:04.760
You might not realize it,

00:01:04.760 --> 00:01:07.000
but the sound of my voice right now

00:01:07.000 --> 00:01:09.700
is actually just a traveling compression,

00:01:09.700 --> 00:01:12.270
or disturbance in the air
that is getting to your ear.

00:01:12.270 --> 00:01:13.860
And that little hairs in your ears

00:01:13.860 --> 00:01:16.980
can sense those changes
in pressure from the air

00:01:16.980 --> 00:01:19.820
and your mind perceives that as sound.

00:01:19.820 --> 00:01:23.010
And once again, this is
a traveling disturbance.

00:01:23.010 --> 00:01:24.740
You have particles that
have high pressure,

00:01:24.740 --> 00:01:26.880
and then they knock into
the particles next to them,

00:01:26.880 --> 00:01:29.090
that then knock into the
particles next to them.

00:01:29.090 --> 00:01:31.430
So if you were to be able to
observe this in slow motion,

00:01:31.430 --> 00:01:34.230
you would see these high
pressure parts right over here

00:01:34.230 --> 00:01:36.450
could be traveling, say to the right.

00:01:36.450 --> 00:01:38.440
And even though this
might be a pressure wave

00:01:38.440 --> 00:01:39.950
that's traveling through the air,

00:01:39.950 --> 00:01:41.530
we can represent it in a way

00:01:41.530 --> 00:01:43.530
that looks a lot like our first rope

00:01:43.530 --> 00:01:45.490
that we were moving up and down.

00:01:45.490 --> 00:01:48.720
Areas where things are
high, in the sound example,

00:01:48.720 --> 00:01:49.740
that's high pressure,

00:01:49.740 --> 00:01:51.510
and you have areas where things are low,

00:01:51.510 --> 00:01:54.110
in the sound example,
that is low pressure.

00:01:54.110 --> 00:01:56.880
Now, when we talk about waves,
there are common properties.

00:01:56.880 --> 00:01:58.700
For example, we might wanna know,

00:01:58.700 --> 00:02:00.500
how much are we getting disturbed

00:02:00.500 --> 00:02:02.440
from what we would call the equilibrium?

00:02:02.440 --> 00:02:03.760
You could view that as maybe

00:02:03.760 --> 00:02:05.840
the middle state right over there.

00:02:05.840 --> 00:02:08.450
Well, if we're getting
disturbed that much,

00:02:08.450 --> 00:02:10.500
we could call that the amplitude.

00:02:10.500 --> 00:02:13.280
That's how much we are going above

00:02:13.280 --> 00:02:15.610
or below that equilibrium.

00:02:15.610 --> 00:02:18.370
This would be the amplitude as well.

00:02:18.370 --> 00:02:20.720
We could think about how far is it

00:02:20.720 --> 00:02:22.870
from the same points on the wave.

00:02:22.870 --> 00:02:26.010
So if we go from one peak to another peak,

00:02:26.010 --> 00:02:28.870
well, we could call that the wavelength,

00:02:28.870 --> 00:02:31.770
and you could just do it from
any one point on the wave

00:02:31.770 --> 00:02:33.840
that's just like it on the wave again.

00:02:33.840 --> 00:02:35.840
So that would be the same wavelength

00:02:35.840 --> 00:02:38.240
as our original wavelength
right over there.

00:02:38.240 --> 00:02:41.720
You might hear the term
frequency of a wave.

00:02:41.720 --> 00:02:43.450
And one way to think about that

00:02:43.450 --> 00:02:46.470
is if you were to just
observe our original rope,

00:02:46.470 --> 00:02:47.407
and if you were to say,

00:02:47.407 --> 00:02:50.770
"How many times does it go all the way up,

00:02:50.770 --> 00:02:52.970
all the way down, and then back up.

00:02:52.970 --> 00:02:54.870
so it completes a full cycle?

00:02:54.870 --> 00:02:57.190
How many times can it do that in a second?

00:02:57.190 --> 00:02:59.760
If it does that five times in second,

00:02:59.760 --> 00:03:02.220
then someone might say it has a frequency

00:03:02.220 --> 00:03:04.650
of five cycles per second.

00:03:04.650 --> 00:03:07.000
Now, everything that we
have just talked about,

00:03:07.000 --> 00:03:09.400
these are called mechanical waves.

00:03:09.400 --> 00:03:10.560
It's a special category,

00:03:10.560 --> 00:03:13.200
probably the ones that
you will see most often.

00:03:13.200 --> 00:03:17.100
Now, mechanical waves need
a medium to travel through.

00:03:17.100 --> 00:03:19.610
In the rope example,
the medium was the rope.

00:03:19.610 --> 00:03:21.440
In the water example, it's the water.

00:03:21.440 --> 00:03:24.950
In the sound example,
the medium is the air.

00:03:24.950 --> 00:03:28.060
Now, there are things that
can be described as waves

00:03:28.060 --> 00:03:30.200
that don't need a medium.

00:03:30.200 --> 00:03:31.280
In particular,

00:03:31.280 --> 00:03:33.300
and this is kind of mind boggling,

00:03:33.300 --> 00:03:36.460
is that light can be considered a wave.

00:03:36.460 --> 00:03:39.340
If we think about the
different frequencies of light,

00:03:39.340 --> 00:03:42.970
our brain perceives that
as different colors.

00:03:42.970 --> 00:03:45.830
And if we think about
the amplitude of light,

00:03:45.830 --> 00:03:49.150
our brain perceives that
as the intensity of light,

00:03:49.150 --> 00:03:50.300
how bright it is.

00:03:50.300 --> 00:03:51.980
And even more mindblowing,

00:03:51.980 --> 00:03:54.780
visible light are just certain frequencies

00:03:54.780 --> 00:03:57.690
of what we would call
electromagnetic waves.

00:03:57.690 --> 00:04:01.270
There's actually higher frequencies
of electromagnetic waves

00:04:01.270 --> 00:04:03.470
that have all sorts of applications.

00:04:03.470 --> 00:04:05.290
You might have heard of ultraviolet light,

00:04:05.290 --> 00:04:07.770
or x-rays, or gamma rays.

00:04:07.770 --> 00:04:09.900
Similarly, there's lower
wavelengths of light.

00:04:09.900 --> 00:04:12.770
You might have heard things
like infrared, or radio waves.

00:04:12.770 --> 00:04:14.670
These are all just different frequencies

00:04:14.670 --> 00:04:17.903
of what's known as electromagnetic waves.


Kinetic energy

WEBVTT
Kind: captions
Language: en

00:00:00.200 --> 00:00:01.120
- [Instructor] Hello, everyone.

00:00:01.120 --> 00:00:04.050
Let's talk about kinetic energy.

00:00:04.050 --> 00:00:06.580
Now, kinetic might be an unfamiliar word,

00:00:06.580 --> 00:00:08.170
but it just comes from a Greek word

00:00:08.170 --> 00:00:10.050
that means of motion,

00:00:10.050 --> 00:00:13.920
so kinetic energy is energy from motion.

00:00:13.920 --> 00:00:16.760
Any massive object that is in motion then

00:00:16.760 --> 00:00:20.280
has kinetic energy, but how much?

00:00:20.280 --> 00:00:23.040
First, let's consider some comparisons.

00:00:23.040 --> 00:00:26.930
This nice rat family, papa,
mama, brother, and sister

00:00:26.930 --> 00:00:29.270
are sitting down to dinner at a long table

00:00:29.270 --> 00:00:31.760
passing blocks of cheese back-and-forth.

00:00:31.760 --> 00:00:33.650
Papa rat asks for the cheddar cheese

00:00:33.650 --> 00:00:36.020
and there are two identical blocks.

00:00:36.020 --> 00:00:38.950
Brother rat pushes one and
sister rat pushes the other,

00:00:38.950 --> 00:00:40.930
so that the second cheese is traveling

00:00:40.930 --> 00:00:43.320
twice as fast as the first cheese.

00:00:43.320 --> 00:00:44.770
Which piece of cheddar cheese

00:00:44.770 --> 00:00:47.633
do you think has more kinetic energy?

00:00:49.090 --> 00:00:51.990
Yes, it's the one going faster.

00:00:51.990 --> 00:00:55.080
Now, papa rat doesn't need
both pieces of cheddar,

00:00:55.080 --> 00:00:57.360
so he eats one and sends one back,

00:00:57.360 --> 00:00:59.410
along with this small piece of Swiss

00:00:59.410 --> 00:01:02.150
that weighs half as much
as the piece of cheddar.

00:01:02.150 --> 00:01:04.490
Papa rat has better
manners than his children,

00:01:04.490 --> 00:01:07.660
so he sends them both
back at the same speed.

00:01:07.660 --> 00:01:09.470
Which piece of cheese would you think

00:01:09.470 --> 00:01:11.333
has more kinetic energy now?

00:01:14.020 --> 00:01:16.770
Yes, the heavier or more massive object,

00:01:16.770 --> 00:01:20.100
in this case, the cheddar,
will have more kinetic energy.

00:01:20.100 --> 00:01:22.380
Let's make it a little more complicated.

00:01:22.380 --> 00:01:24.120
Brother and sister rat are full,

00:01:24.120 --> 00:01:27.040
so they send the cheeses
back for mama rat.

00:01:27.040 --> 00:01:29.340
Brother rat pushes the
larger piece of cheddar

00:01:29.340 --> 00:01:31.990
and sister rat pushes the
smaller piece of Swiss

00:01:31.990 --> 00:01:36.210
so that the Swiss is going
twice as fast as the cheddar.

00:01:36.210 --> 00:01:38.673
Now, which cheese has more kinetic energy?

00:01:40.870 --> 00:01:44.440
In fact, it turns out that it's
the Swiss in this scenario.

00:01:44.440 --> 00:01:47.570
Kinetic energy depends
on both mass and speed,

00:01:47.570 --> 00:01:50.800
but the dependence on speed is stronger.

00:01:50.800 --> 00:01:53.110
This estimation of kinetic energy

00:01:53.110 --> 00:01:55.290
can be quantified in an equation

00:01:55.290 --> 00:01:58.170
that lets us calculate
kinetic energy exactly.

00:01:58.170 --> 00:02:00.500
We said kinetic energy depends on the mass

00:02:00.500 --> 00:02:03.740
and the speed, which we'll
write as V for velocity,

00:02:03.740 --> 00:02:07.440
so we can start with K E equals M times V.

00:02:07.440 --> 00:02:10.580
But we said that it
depends more on the speed,

00:02:10.580 --> 00:02:14.020
so the velocity here is actually squared.

00:02:14.020 --> 00:02:16.830
This means that if an
object's mass doubles,

00:02:16.830 --> 00:02:19.420
its kinetic energy also doubles,

00:02:19.420 --> 00:02:21.280
but if its speed doubles,

00:02:21.280 --> 00:02:24.160
the kinetic energy actually quadruples.

00:02:24.160 --> 00:02:26.180
And there's also a constant factor of 1/2

00:02:26.180 --> 00:02:27.530
at the beginning of the equation,

00:02:27.530 --> 00:02:29.380
but we won't go into
the details of the math

00:02:29.380 --> 00:02:30.750
of deriving this today.

00:02:30.750 --> 00:02:33.370
So, this is the equation
for kinetic energy,

00:02:33.370 --> 00:02:36.290
1/2 M V squared.

00:02:36.290 --> 00:02:39.340
Let's apply this equation
to our cheesy example.

00:02:39.340 --> 00:02:42.384
Say the Swiss has a mass of .05 kilograms,

00:02:42.384 --> 00:02:45.510
which makes the cheddar's
mass .1 kilograms.

00:02:45.510 --> 00:02:48.240
When both cheeses have the same speed,

00:02:48.240 --> 00:02:50.060
say two meters per second,

00:02:50.060 --> 00:02:53.642
the cheddar's kinetic energy
is 1/2 times .1 kilograms,

00:02:53.642 --> 00:02:57.708
times two meters per second
squared, which is .2 Joules.

00:02:57.708 --> 00:03:01.829
The Swiss's kinetic energy
is 1/2 times .05 kilograms

00:03:01.829 --> 00:03:05.073
times two meters per second
squared, which is .1 Joules,

00:03:05.073 --> 00:03:07.550
or half the kinetic energy of the cheddar.

00:03:07.550 --> 00:03:09.697
So we can see that at the same speed,

00:03:09.697 --> 00:03:11.677
the cheddar has more kinetic energy

00:03:11.677 --> 00:03:14.177
because it has more mass.

00:03:14.177 --> 00:03:17.293
But when the Swiss has a speed
of four meters per second

00:03:17.293 --> 00:03:20.030
and the cheddar still has a
speed of two meters per second,

00:03:20.030 --> 00:03:24.740
the Swiss's kinetic energy is
now 1/2 times .05 kilograms

00:03:24.740 --> 00:03:29.560
times four meters per second
squared, which is .4 Joules.

00:03:29.560 --> 00:03:31.480
So now, the kinetic energy of the Swiss

00:03:31.480 --> 00:03:33.900
is twice the kinetic
energy of the cheddar.

00:03:33.900 --> 00:03:36.730
So we can see that even though
the cheddar has more mass

00:03:36.730 --> 00:03:38.510
the Swiss has more kinetic energy

00:03:38.510 --> 00:03:41.080
because it's going faster.

00:03:41.080 --> 00:03:42.660
In summary, kinetic energy

00:03:42.660 --> 00:03:45.370
is the motion energy of an object.

00:03:45.370 --> 00:03:48.740
The equation for kinetic
energy is 1/2 M V squared.

00:03:48.740 --> 00:03:52.060
So as mass increases,
kinetic energy increases,

00:03:52.060 --> 00:03:55.530
like the more massive
cheddar versus the Swiss,

00:03:55.530 --> 00:03:57.230
and as velocity increases,

00:03:57.230 --> 00:03:59.480
kinetic energy increases even more,

00:03:59.480 --> 00:04:03.040
like the speedy Swiss
versus the slower cheddar.

00:04:03.040 --> 00:04:03.873
Thanks for watching,

00:04:03.873 --> 00:04:05.983
and I hope you learned a
little bit of something.


Electromagnetism

WEBVTT
Kind: captions
Language: en

00:00:00.130 --> 00:00:01.380
- [Instructor] You know what a magnet is,

00:00:01.380 --> 00:00:05.180
but did you know that some
magnets can be turned on and off?

00:00:05.180 --> 00:00:09.200
One type of temporary magnet
is called an electromagnet.

00:00:09.200 --> 00:00:12.010
So what is electromagnetism?

00:00:12.010 --> 00:00:14.750
Well, the hint is in the name itself.

00:00:14.750 --> 00:00:17.520
Electro for electrical,

00:00:17.520 --> 00:00:21.260
and magnet for, well, magnet.

00:00:21.260 --> 00:00:23.670
Let's take a moment to
look at the definition

00:00:23.670 --> 00:00:25.860
of what an electromagnet is.

00:00:25.860 --> 00:00:29.630
Electromagnets are materials
that become magnets

00:00:29.630 --> 00:00:31.593
in the presence of electricity.

00:00:32.460 --> 00:00:34.320
But how does that even happen?

00:00:34.320 --> 00:00:35.380
Well, it turns out

00:00:35.380 --> 00:00:38.100
that electrically-charged
particles in motion

00:00:38.100 --> 00:00:41.280
actually have small
magnetic fields around them.

00:00:41.280 --> 00:00:45.530
So, if we run electricity through a wire,

00:00:45.530 --> 00:00:48.303
a magnetic field will be
created around the wire.

00:00:49.160 --> 00:00:52.500
Now, we can control the
strength of this magnetic field

00:00:52.500 --> 00:00:54.320
in a couple of ways.

00:00:54.320 --> 00:00:57.680
We can move more electric
charges through the wire

00:00:57.680 --> 00:00:58.933
at a faster rate,

00:00:59.850 --> 00:01:03.313
and we do this by increasing
the electrical current.

00:01:04.720 --> 00:01:08.800
The second way is to increase the density

00:01:08.800 --> 00:01:10.223
of the charged particles,

00:01:11.740 --> 00:01:16.010
and we can do this by
looping the wire into a coil.

00:01:16.010 --> 00:01:18.110
This gives us more charged particles

00:01:18.110 --> 00:01:20.810
with magnetic fields in a small space,

00:01:20.810 --> 00:01:23.210
strengthening the magnetic force.

00:01:23.210 --> 00:01:26.150
The other thing we can
control with electromagnets

00:01:26.150 --> 00:01:29.790
is the direction of the magnetic field,

00:01:29.790 --> 00:01:32.370
and we can do this by
changing the direction

00:01:32.370 --> 00:01:34.170
of the electricity.

00:01:34.170 --> 00:01:37.400
So if we go back to this
wire example from earlier

00:01:37.400 --> 00:01:38.760
and change the direction

00:01:38.760 --> 00:01:41.370
of the electricity
running through that wire,

00:01:41.370 --> 00:01:45.500
well, the magnetic fields
will also change direction.

00:01:45.500 --> 00:01:48.010
This makes electromagnets quite different

00:01:48.010 --> 00:01:49.890
from permanent magnets.

00:01:49.890 --> 00:01:51.510
So let's take a look at that

00:01:51.510 --> 00:01:55.933
and compare permanent
magnets to electromagnets.

00:01:58.110 --> 00:02:00.630
Electromagnets are typically made

00:02:00.630 --> 00:02:03.210
of loops of wire and a coil.

00:02:03.210 --> 00:02:06.010
The wire is typically
made of metal like copper

00:02:06.010 --> 00:02:07.840
and wrapped around pieces of metal

00:02:07.840 --> 00:02:10.680
like iron, nickel, or cobalt.

00:02:10.680 --> 00:02:12.980
This is different from a permanent magnet

00:02:12.980 --> 00:02:15.603
because permanent magnets
don't need this wire.

00:02:16.820 --> 00:02:19.530
Permanent magnets also have fixed poles.

00:02:19.530 --> 00:02:23.140
You can't change the north and
south poles on these magnets,

00:02:23.140 --> 00:02:25.900
but as we now know, for electromagnets,

00:02:25.900 --> 00:02:28.870
we can change these poles
by changing the direction

00:02:28.870 --> 00:02:30.570
of the electrical current.

00:02:30.570 --> 00:02:33.450
So, if we have an electromagnet
with a north and south pole

00:02:33.450 --> 00:02:34.440
that looks like this

00:02:34.440 --> 00:02:36.890
and a current flowing in this direction,

00:02:36.890 --> 00:02:38.860
well, we can change the poles

00:02:38.860 --> 00:02:40.633
and the direction of the current.

00:02:42.110 --> 00:02:45.050
Permanent magnets have a fixed strength,

00:02:45.050 --> 00:02:46.480
but we just talked about

00:02:46.480 --> 00:02:49.270
how we can change the
strength of electromagnets,

00:02:49.270 --> 00:02:52.600
so electromagnets have
adjustable strength.

00:02:52.600 --> 00:02:55.900
And finally, electromagnets
need a power source

00:02:55.900 --> 00:02:58.130
in order to generate
the electricity required

00:02:58.130 --> 00:03:00.140
to produce magnetic fields.

00:03:00.140 --> 00:03:03.240
Permanent magnets do
not need a power source,

00:03:03.240 --> 00:03:06.550
but this means that we can
also turn electromagnets

00:03:06.550 --> 00:03:07.690
on and off,

00:03:07.690 --> 00:03:10.030
which is pretty cool
when you think about it.

00:03:10.030 --> 00:03:10.863
On the other hand,

00:03:10.863 --> 00:03:13.650
permanent magnets are always on.

00:03:13.650 --> 00:03:15.680
Now, you might be thinking

00:03:15.680 --> 00:03:19.220
if electrical charge can affect magnetism,

00:03:19.220 --> 00:03:23.060
can magnetism affect electrical charge?

00:03:23.060 --> 00:03:26.590
Absolutely, let's look
at how we can do that.

00:03:26.590 --> 00:03:30.930
The only way to do this is by
changing the magnetic field

00:03:30.930 --> 00:03:32.860
around the charged particles.

00:03:32.860 --> 00:03:36.070
This can be done by moving magnets closer

00:03:36.070 --> 00:03:38.650
or further away from the particles

00:03:38.650 --> 00:03:41.520
or by spinning the magnets.

00:03:41.520 --> 00:03:45.610
In fact, spinning magnets is
how most of the electricity

00:03:45.610 --> 00:03:48.380
we use in cities and homes is generated.

00:03:48.380 --> 00:03:51.260
A turbine spins a magnet inside a coil

00:03:51.260 --> 00:03:52.890
to produce electricity,

00:03:52.890 --> 00:03:55.750
and since electromagnets
need a power source,

00:03:55.750 --> 00:03:58.690
this turbine is powered by wind.

00:03:58.690 --> 00:04:00.870
So you can see why electromagnetism

00:04:00.870 --> 00:04:02.840
is an incredibly important force,

00:04:02.840 --> 00:04:06.160
and this isn't the only
important application of it.

00:04:06.160 --> 00:04:09.330
We use electromagnets in all
sorts of other applications,

00:04:09.330 --> 00:04:13.423
from motors to speakers
and even medical scanners.


Fields

WEBVTT
Kind: captions
Language: en

00:00:00.350 --> 00:00:02.890
- If you hold a ball up
in the air and let it go,

00:00:02.890 --> 00:00:05.890
you know it's going to fall, but why?

00:00:05.890 --> 00:00:08.190
Nothing is touching it once you let it go.

00:00:08.190 --> 00:00:10.330
How can there be a force on it?

00:00:10.330 --> 00:00:13.170
Well, this is because
Earth's gravitational force

00:00:13.170 --> 00:00:17.210
is pulling the ball and
gravity is a non-contact force.

00:00:17.210 --> 00:00:19.940
Non-contact forces don't
have to touch an object

00:00:19.940 --> 00:00:21.710
to exert a force on it.

00:00:21.710 --> 00:00:24.840
Instead, these forces act over a region.

00:00:24.840 --> 00:00:27.100
So if an object is in that region,

00:00:27.100 --> 00:00:29.250
it will be affected by the force.

00:00:29.250 --> 00:00:32.720
In this case, the ball is in
Earth's gravitational field,

00:00:32.720 --> 00:00:35.880
and so it feels an attractive
force towards the Earth

00:00:35.880 --> 00:00:37.852
and the ball falls to the ground.

00:00:37.852 --> 00:00:41.350
Field forces include non-contact forces,

00:00:41.350 --> 00:00:45.518
such as electric, magnetic,

00:00:45.518 --> 00:00:49.270
and of course, gravitational forces.

00:00:49.270 --> 00:00:51.700
So since these forces are non-contact,

00:00:51.700 --> 00:00:54.340
they can exert a force on
objects they aren't touching,

00:00:54.340 --> 00:00:55.960
but how do these objects know

00:00:55.960 --> 00:00:57.950
if there's a force between them?

00:00:57.950 --> 00:01:00.380
To explain these non-contact forces,

00:01:00.380 --> 00:01:02.490
scientists eventually developed the idea

00:01:02.490 --> 00:01:04.340
that these objects were
surrounded by something

00:01:04.340 --> 00:01:05.513
called a field.

00:01:06.420 --> 00:01:08.230
So what is a field?

00:01:08.230 --> 00:01:10.860
A field extends through
space from an object

00:01:10.860 --> 00:01:12.720
with certain physical properties.

00:01:12.720 --> 00:01:14.330
What are those?

00:01:14.330 --> 00:01:17.150
Well, for gravitational forces,

00:01:17.150 --> 00:01:19.650
these affect objects with mass.

00:01:19.650 --> 00:01:23.241
So any object with mass
has a gravitational field

00:01:23.241 --> 00:01:27.360
surrounding it that points
towards the object's center.

00:01:27.360 --> 00:01:29.430
The further you move away from the object,

00:01:29.430 --> 00:01:32.900
the less dense the field and
weaker the field becomes.

00:01:32.900 --> 00:01:36.080
Electric forces effect charged objects.

00:01:36.080 --> 00:01:40.470
So an electric field surrounds
any object with a net charge,

00:01:40.470 --> 00:01:44.469
and the direction of this field
will depend on the charge.

00:01:44.469 --> 00:01:47.510
Magnetic fields will affect magnets

00:01:47.510 --> 00:01:50.173
and any other material
with magnetic properties.

00:01:51.410 --> 00:01:55.136
Each spot on a field has two
things associated with it:

00:01:55.136 --> 00:01:58.700
magnitude and direction.

00:01:58.700 --> 00:01:59.890
And these help us predict

00:01:59.890 --> 00:02:02.000
what forces objects will experience

00:02:02.000 --> 00:02:03.760
when they're in the field.

00:02:03.760 --> 00:02:07.530
So let's look at an example
to help understand this.

00:02:07.530 --> 00:02:09.700
Say we have a planet.

00:02:09.700 --> 00:02:12.470
Now, the planet has a lot of mass

00:02:12.470 --> 00:02:14.490
so we know it's going to be surrounded

00:02:14.490 --> 00:02:16.159
by a gravitational field

00:02:16.159 --> 00:02:19.790
that points towards the
center of the planet.

00:02:19.790 --> 00:02:21.780
I can draw these little field lines

00:02:21.780 --> 00:02:25.500
that show the direction of
the field and its strength.

00:02:25.500 --> 00:02:27.400
As we move away from the planet,

00:02:27.400 --> 00:02:28.890
the field will start to weaken,

00:02:28.890 --> 00:02:32.470
and I'm going to represent
that by a less dense field

00:02:32.470 --> 00:02:33.930
with these arrows.

00:02:33.930 --> 00:02:35.610
Now, let's say there's an asteroid

00:02:35.610 --> 00:02:38.120
moving near the planet in this direction.

00:02:38.120 --> 00:02:41.265
I know that the asteroid,
as it's shown here,

00:02:41.265 --> 00:02:45.230
is in the outskirts of this
planet's gravitational field.

00:02:45.230 --> 00:02:48.490
So it is going to feel some
gravitational attraction

00:02:48.490 --> 00:02:49.690
towards the planet,

00:02:49.690 --> 00:02:52.960
which we can draw with this vector, Fg,

00:02:52.960 --> 00:02:55.340
which is force of gravity.

00:02:55.340 --> 00:02:57.210
Now, because it's attracted to the planet,

00:02:57.210 --> 00:03:00.310
the astroid will continue
to move towards the planet.

00:03:00.310 --> 00:03:03.070
And the closer the asteroid
gets to the planet,

00:03:03.070 --> 00:03:04.014
the stronger the field

00:03:04.014 --> 00:03:07.412
and the stronger the force
of attraction it will feel.

00:03:07.412 --> 00:03:11.160
And so in this way,
scientists can use fields

00:03:11.160 --> 00:03:13.410
to help predict behavior of objects

00:03:13.410 --> 00:03:16.060
experiencing non-contact forces.

00:03:16.060 --> 00:03:18.180
And all of this may sound kind of odd,

00:03:18.180 --> 00:03:21.269
but you probably already
think about forces this way.

00:03:21.269 --> 00:03:23.910
For example, if we go back to the ball

00:03:23.910 --> 00:03:25.700
that you know is going to fall,

00:03:25.700 --> 00:03:28.430
you knew this because the
force of gravity from Earth

00:03:28.430 --> 00:03:30.960
was going to pull the
ball towards the Earth.

00:03:30.960 --> 00:03:33.430
But now you also know that that's because

00:03:33.430 --> 00:03:36.400
Earth's gravity is a field force.

00:03:36.400 --> 00:03:40.550
And so the ball is in the
field of gravity for Earth

00:03:40.550 --> 00:03:43.950
and experiences an attractive
gravitational force.

00:03:43.950 --> 00:03:46.842
So while fields may sound mysterious,

00:03:46.842 --> 00:03:51.842
they really just mean that a
force is felt over a distance.

00:03:52.060 --> 00:03:54.625
Gravitational, electric,
and magnetic forces

00:03:54.625 --> 00:03:56.400
are all field forces,

00:03:56.400 --> 00:03:58.241
which means they act over distance

00:03:58.241 --> 00:04:00.753
and can affect a lot of objects.


Magnetic forces

WEBVTT
Kind: captions
Language: en

00:00:00.120 --> 00:00:01.350
- [Instructor] Let's talk about magnets

00:00:01.350 --> 00:00:03.310
and magnetic forces.

00:00:03.310 --> 00:00:05.690
Magnets are these neat
objects that are able

00:00:05.690 --> 00:00:08.100
to attract metals like iron.

00:00:08.100 --> 00:00:10.310
Magnets are used in all sorts of things,

00:00:10.310 --> 00:00:13.650
from holding paper on your refrigerator,

00:00:13.650 --> 00:00:17.180
to computers, to compasses.

00:00:17.180 --> 00:00:20.490
So magnets can be used
to stick things together,

00:00:20.490 --> 00:00:23.890
point us in the right
direction, and even lift things,

00:00:23.890 --> 00:00:27.360
and they do this through magnetic forces.

00:00:27.360 --> 00:00:30.930
If you've handled two magnets,
you felt magnetic forces,

00:00:30.930 --> 00:00:33.670
even when the magnets
weren't touching each other.

00:00:33.670 --> 00:00:37.950
That's because magnetic
forces are non-contact forces,

00:00:37.950 --> 00:00:40.660
which just means they
can affect other objects

00:00:40.660 --> 00:00:42.210
they aren't even touching.

00:00:42.210 --> 00:00:46.560
Magnets will attract or repel each other,

00:00:46.560 --> 00:00:51.010
and this attraction or
repulsion is a magnetic force.

00:00:51.010 --> 00:00:54.680
But magnetic forces don't
affect everything the same way.

00:00:54.680 --> 00:00:56.820
Otherwise, a magnet would stick to you,

00:00:56.820 --> 00:00:58.640
not just a refrigerator.

00:00:58.640 --> 00:01:01.680
In this video, we're going to
talk about the magnetic forces

00:01:01.680 --> 00:01:03.313
between two magnets.

00:01:04.200 --> 00:01:06.900
So why do magnet sometimes
attract each other

00:01:06.900 --> 00:01:09.610
and other times repel each other?

00:01:09.610 --> 00:01:14.470
Well, this has to do with the
orientation of the magnets.

00:01:14.470 --> 00:01:17.440
Orientation is really just a fancy word

00:01:17.440 --> 00:01:21.600
for how the magnets are positioned
compared to one another.

00:01:21.600 --> 00:01:24.800
You see, it turns out that
each magnet has a north

00:01:24.800 --> 00:01:26.910
and south pole,

00:01:26.910 --> 00:01:30.870
but what does this have to do
with attraction or repulsion?

00:01:30.870 --> 00:01:33.570
Well, as you may have
heard, opposites attract.

00:01:33.570 --> 00:01:37.490
So, if you face the
north pole of one magnet

00:01:37.490 --> 00:01:41.430
to the south pole of
another magnet, guess what?

00:01:41.430 --> 00:01:43.900
They will be attracted to each other.

00:01:43.900 --> 00:01:46.350
But if you turn one of
those magnets around

00:01:46.350 --> 00:01:49.400
so that you have two north
poles facing each other,

00:01:49.400 --> 00:01:50.940
they will repel.

00:01:50.940 --> 00:01:52.080
And the same thing would happen

00:01:52.080 --> 00:01:54.610
if it was two south
pulse facing each other.

00:01:54.610 --> 00:01:58.630
So the direction of the magnetic
force completely depends

00:01:58.630 --> 00:02:01.150
on the orientation of the magnets.

00:02:01.150 --> 00:02:03.850
Orientation, though, is just one thing

00:02:03.850 --> 00:02:05.710
that affects magnetic forces.

00:02:05.710 --> 00:02:08.420
The strength of magnetic forces depends

00:02:08.420 --> 00:02:10.130
on a couple of things.

00:02:10.130 --> 00:02:12.093
For one, distance.

00:02:13.040 --> 00:02:14.830
If you've ever held two magnets,

00:02:14.830 --> 00:02:17.930
you may have noticed that
when you move them closer,

00:02:17.930 --> 00:02:20.310
they seem to almost jump together.

00:02:20.310 --> 00:02:23.260
Or if you try to push
two light poles together,

00:02:23.260 --> 00:02:25.370
they get harder and
harder to hold together

00:02:25.370 --> 00:02:27.050
the closer you get.

00:02:27.050 --> 00:02:30.740
This is because magnetic
forces depend on distance.

00:02:30.740 --> 00:02:32.840
The closer the two magnets are together,

00:02:32.840 --> 00:02:34.770
the stronger the force between them.

00:02:34.770 --> 00:02:39.063
So, as the distance decreases,
the force increases,

00:02:39.900 --> 00:02:41.970
but the farther away they are,

00:02:41.970 --> 00:02:44.130
the weaker the magnetic force is.

00:02:44.130 --> 00:02:48.610
So, distance increases; force decreases.

00:02:48.610 --> 00:02:50.340
The other big factor that affects

00:02:50.340 --> 00:02:52.330
how strong a magnetic force is...

00:02:52.330 --> 00:02:54.610
Well, the magnets themselves.

00:02:54.610 --> 00:02:56.550
Sub magnets are really weak,

00:02:56.550 --> 00:02:58.750
like a lot of refrigerator magnets.

00:02:58.750 --> 00:03:01.560
Others are so strong that even tiny ones

00:03:01.560 --> 00:03:04.460
can be almost impossible to pull apart.

00:03:04.460 --> 00:03:07.070
Some of these stronger
magnets are even used

00:03:07.070 --> 00:03:10.630
to make high-speed trains
levitate off the ground.

00:03:10.630 --> 00:03:14.410
And yes, this rectangle
is supposed to be a train.

00:03:14.410 --> 00:03:16.830
While my drawing isn't amazing,

00:03:16.830 --> 00:03:20.553
the fact that magnetic forces
can levitate a train is.


Gravitational forces

WEBVTT
Kind: captions
Language: en

00:00:00.130 --> 00:00:02.120
- [Instructor] When you
hear the word gravity,

00:00:02.120 --> 00:00:04.430
you probably just think of things falling,

00:00:04.430 --> 00:00:06.610
like an apple from a tree.

00:00:06.610 --> 00:00:08.240
But did you know it's also the reason

00:00:08.240 --> 00:00:10.910
why your lamp is staying on the floor?

00:00:10.910 --> 00:00:13.000
That's because gravity is so much more

00:00:13.000 --> 00:00:14.730
than things falling down.

00:00:14.730 --> 00:00:17.089
Gravitational forces are
these invisible forces

00:00:17.089 --> 00:00:20.260
that pull objects together.

00:00:20.260 --> 00:00:22.880
So gravitational force
is actually attracting

00:00:22.880 --> 00:00:25.819
the lamp to the floor
and these forces exist

00:00:25.819 --> 00:00:28.470
between all objects with mass.

00:00:28.470 --> 00:00:29.771
So let's write these key points out

00:00:29.771 --> 00:00:32.630
about gravitational
forces, which I'm going

00:00:32.630 --> 00:00:34.403
to use GF to represent.

00:00:35.380 --> 00:00:38.720
We said they are attractive forces,

00:00:38.720 --> 00:00:43.720
and that they exist between
all objects with mass.

00:00:44.630 --> 00:00:49.630
Objects with mass, to explain this,

00:00:50.100 --> 00:00:52.810
we first need to remember
a couple of things.

00:00:52.810 --> 00:00:57.000
Mass is how much matter objects have,

00:00:57.000 --> 00:01:00.590
and matter is the stuff
an object is made of.

00:01:00.590 --> 00:01:03.680
Any object with mass generates
a gravitational pull.

00:01:03.680 --> 00:01:05.710
So, there is a gravitational force

00:01:05.710 --> 00:01:08.720
of attraction between every object.

00:01:08.720 --> 00:01:10.390
The amount of gravitational force

00:01:10.390 --> 00:01:14.223
between two objects will
depend on two things:

00:01:15.420 --> 00:01:20.203
the masses of the two objects
and the distance between them.

00:01:21.470 --> 00:01:23.950
The mass of each object is proportional

00:01:23.950 --> 00:01:26.130
to the gravitational force.

00:01:26.130 --> 00:01:29.550
This means that the
more mass an object has

00:01:29.550 --> 00:01:31.993
the stronger it's gravitational force.

00:01:33.010 --> 00:01:36.470
And, now, we can understand
why gravity makes things fall.

00:01:36.470 --> 00:01:39.380
The Earth has massive, literally.

00:01:39.380 --> 00:01:43.400
It's almost six septillion kilograms.

00:01:43.400 --> 00:01:48.080
That's a 6 with 24 zeros after it.

00:01:48.080 --> 00:01:50.573
So, it generates a huge attractive force.

00:01:51.720 --> 00:01:55.160
For comparison, my lamp
is only one kilogram,

00:01:55.160 --> 00:01:57.630
which is why if I jump, I fall towards

00:01:57.630 --> 00:01:59.613
the Earth and not towards my lamp.

00:02:00.720 --> 00:02:03.070
But we said the mass of the object

00:02:03.070 --> 00:02:04.670
is just one factor affecting

00:02:04.670 --> 00:02:07.310
the strength of its gravitational force.

00:02:07.310 --> 00:02:10.040
The other is the distance between objects.

00:02:10.040 --> 00:02:12.450
The more distance between the objects

00:02:12.450 --> 00:02:14.993
the weaker the gravitational
pull between them.

00:02:16.210 --> 00:02:18.680
For small objects without much mass,

00:02:18.680 --> 00:02:21.690
it doesn't take much distance
for their gravitational forces

00:02:21.690 --> 00:02:25.023
between each other to be so
weak that we don't notice them.

00:02:26.210 --> 00:02:28.030
For something like the Earth, you have

00:02:28.030 --> 00:02:31.560
to go really far away to not be affected

00:02:31.560 --> 00:02:34.100
by its gravitational force of attraction.

00:02:34.100 --> 00:02:39.100
I mean, look at the moon, it's
almost 240,000 miles away,

00:02:39.900 --> 00:02:43.620
that's almost 400,000 kilometers away,

00:02:43.620 --> 00:02:46.970
and it still feels effects
from Earth's gravity.

00:02:46.970 --> 00:02:50.060
That's why it's orbiting
us, but since the moon

00:02:50.060 --> 00:02:53.830
is also a pretty massive
object, we do experience

00:02:53.830 --> 00:02:57.320
the effects of its
gravitational pull on the Earth.

00:02:57.320 --> 00:03:00.740
This is why we have tides,
the moon's gravitational force

00:03:00.740 --> 00:03:03.080
will pull on Earth's water, which results

00:03:03.080 --> 00:03:05.073
in us having high and low tides.

00:03:06.100 --> 00:03:08.980
Now, you might be wondering if
gravity can affect the moon,

00:03:08.980 --> 00:03:12.470
or cause tides, how can
we even move around?

00:03:12.470 --> 00:03:14.230
Why aren't we just face
planted on the ground

00:03:14.230 --> 00:03:17.280
because Earth's gravity
is pulling us towards it?

00:03:17.280 --> 00:03:21.050
It turns out that actually
gravity is a pretty weak force.

00:03:21.050 --> 00:03:23.270
We only even notice its
effects when an object

00:03:23.270 --> 00:03:26.000
is massive, like planets or stars,

00:03:26.000 --> 00:03:28.150
and the gravitational force on you

00:03:28.150 --> 00:03:32.420
is way weaker than most
forces you exert every day.

00:03:32.420 --> 00:03:35.490
In fact, every time you
pick up a glass of water,

00:03:35.490 --> 00:03:38.490
you're overpowering the
entire mass of Earth.

00:03:38.490 --> 00:03:39.553
How cool is that?


Action and reaction forces

WEBVTT
Kind: captions
Language: en

00:00:00.300 --> 00:00:02.160
- [Lecturer] You've probably
heard the phrase that

00:00:02.160 --> 00:00:03.530
for every force,

00:00:03.530 --> 00:00:06.340
there's an equal and
opposite reaction force.

00:00:06.340 --> 00:00:10.200
And this is also known as
Newton's third law of motion,

00:00:10.200 --> 00:00:14.240
but it's also one of the most
misunderstood laws of physics.

00:00:14.240 --> 00:00:16.330
So that's why we're going
to dig into it a little bit

00:00:16.330 --> 00:00:17.930
in this video.

00:00:17.930 --> 00:00:21.700
So I have two examples here
where Newton's third law

00:00:21.700 --> 00:00:24.570
or this notion of an
action and a reaction force

00:00:24.570 --> 00:00:25.960
is happening.

00:00:25.960 --> 00:00:27.960
So over here, you have this plane flying

00:00:27.960 --> 00:00:30.620
and the plane is able to move forward

00:00:30.620 --> 00:00:33.960
by pushing air particles
through these jet engines.

00:00:33.960 --> 00:00:36.760
So these air particles are pushed outward

00:00:36.760 --> 00:00:41.280
at a very, very high velocity
out the back of the engines.

00:00:41.280 --> 00:00:43.800
If you are to enlarge one
of those air particles,

00:00:43.800 --> 00:00:46.790
let's say, this is this
purple dot right over here.

00:00:46.790 --> 00:00:49.530
There is a force that
is being exerted on it

00:00:49.530 --> 00:00:51.080
by the jet engine.

00:00:51.080 --> 00:00:55.240
And that force is going in that direction.

00:00:55.240 --> 00:00:58.700
So what is the equal and
opposite reaction force?

00:00:58.700 --> 00:01:01.100
Well, the equal and
opposite reaction force

00:01:01.100 --> 00:01:04.390
is not also are occurring
on that molecule.

00:01:04.390 --> 00:01:07.320
It's what the molecule
is doing to the plane.

00:01:07.320 --> 00:01:09.790
The equal and opposite reaction force

00:01:09.790 --> 00:01:11.813
is that the molecule
is going to be pushing

00:01:11.813 --> 00:01:16.110
on the jet engine with an
equal but an opposite force.

00:01:16.110 --> 00:01:18.670
So it's going to go in
the opposite direction.

00:01:18.670 --> 00:01:22.380
And that's how the jet is
able to accelerate forward

00:01:22.380 --> 00:01:25.190
by pushing on these particles
and accelerating them

00:01:25.190 --> 00:01:27.530
backward by exerting a force on them.

00:01:27.530 --> 00:01:30.140
The equal and opposite force is the force

00:01:30.140 --> 00:01:31.210
that the particles,

00:01:31.210 --> 00:01:33.930
those molecules of air
are exerting on the jet

00:01:33.930 --> 00:01:35.810
and moving it forward.

00:01:35.810 --> 00:01:39.380
The same thing here is
going on with this rocket.

00:01:39.380 --> 00:01:42.280
You have some rocket fuel in there.

00:01:42.280 --> 00:01:43.210
It gets ignited.

00:01:43.210 --> 00:01:44.360
It explodes.

00:01:44.360 --> 00:01:46.070
And as it explodes,

00:01:46.070 --> 00:01:51.070
there's a force that exerts
on those little molecules

00:01:51.170 --> 00:01:55.113
and that force is going in this direction.

00:01:56.290 --> 00:01:57.400
But as it does that,

00:01:57.400 --> 00:01:59.620
there's an equal and opposite force

00:01:59.620 --> 00:02:03.360
that the molecules are
exerting on the rocket.

00:02:03.360 --> 00:02:06.950
The rocket is having a force acted on it,

00:02:06.950 --> 00:02:10.010
once again, equal and opposite.

00:02:10.010 --> 00:02:12.810
So it's important to realize
that the reaction force

00:02:12.810 --> 00:02:14.680
is not on the same object.

00:02:14.680 --> 00:02:16.040
It's on the other object.

00:02:16.040 --> 00:02:19.700
If one object is putting
an action force on another,

00:02:19.700 --> 00:02:22.200
then the second object is
putting a reaction force

00:02:22.200 --> 00:02:23.580
on the first.

00:02:23.580 --> 00:02:26.730
The forces do not cancel out.

00:02:26.730 --> 00:02:28.480
It's also important to realize that

00:02:28.480 --> 00:02:31.000
both forces are generated in pairs

00:02:31.000 --> 00:02:33.490
and happen at the exact same time.

00:02:33.490 --> 00:02:35.320
There's no delay.

00:02:35.320 --> 00:02:37.720
We can look at other examples of this.

00:02:37.720 --> 00:02:40.620
This is a scenario that I would
never want to be caught in

00:02:40.620 --> 00:02:42.010
being drifting through space.

00:02:42.010 --> 00:02:44.870
Now this astronaut here has
some type of a rocket pack

00:02:44.870 --> 00:02:46.030
that might help him move around.

00:02:46.030 --> 00:02:49.170
But let's say that your
rocket pack ran out of fuel

00:02:49.170 --> 00:02:51.160
and you're just drifting through space.

00:02:51.160 --> 00:02:54.563
How can you get back to your spaceship?

00:02:55.440 --> 00:02:57.710
Well, if you have a
wrench or something on you

00:02:57.710 --> 00:02:59.150
that you can throw,

00:02:59.150 --> 00:03:01.683
if you can take that wrench,

00:03:03.070 --> 00:03:06.400
and if you can push that
wrench in that direction,

00:03:06.400 --> 00:03:09.310
and let's say your spaceship
is over here to the left.

00:03:09.310 --> 00:03:11.900
Well, the equal and
opposite force is the force

00:03:11.900 --> 00:03:14.880
that the wrench is going to
exert on you, the astronaut,

00:03:14.880 --> 00:03:17.840
and then it will push
you in that direction

00:03:17.840 --> 00:03:20.160
and accelerate you in that direction.

00:03:20.160 --> 00:03:21.230
So that's a useful thing,

00:03:21.230 --> 00:03:23.930
if you're ever get caught
drifting through space.

00:03:23.930 --> 00:03:25.820
But you could do an experiment right now.

00:03:25.820 --> 00:03:28.210
Press on the table in front of you.

00:03:28.210 --> 00:03:29.630
When you press on that table,

00:03:29.630 --> 00:03:32.660
you're clearly putting
a force onto that table.

00:03:32.660 --> 00:03:35.660
If your table is soft, you
will see it get compressed.

00:03:35.660 --> 00:03:38.880
But notice your finger itself
is also getting compressed.

00:03:38.880 --> 00:03:40.380
And the whole reason why
you can even feel that

00:03:40.380 --> 00:03:42.040
is because your finger
is getting compressed.

00:03:42.040 --> 00:03:43.910
And that is the equal and opposite force

00:03:43.910 --> 00:03:46.740
that the table is putting on your finger.

00:03:46.740 --> 00:03:50.570
And this can happen at very,
very large distances as well.

00:03:50.570 --> 00:03:53.350
The whole reason why the Moon
is in orbit around the Earth

00:03:53.350 --> 00:03:56.330
is because there's a gravitational force

00:03:56.330 --> 00:03:59.260
of Earth's mass acting on the Moon.

00:03:59.260 --> 00:04:03.610
But there's an equal and
opposite force of the Moon

00:04:03.610 --> 00:04:05.987
acting on Earth.

00:04:05.987 --> 00:04:08.510
And it's actually not
that the Moon is rotating

00:04:08.510 --> 00:04:09.343
around the Earth.

00:04:09.343 --> 00:04:11.280
It's actually, they're
both rotating around

00:04:11.280 --> 00:04:14.000
the center of mass of their combination,

00:04:14.000 --> 00:04:16.530
that just happens to be
so much closer to Earth.

00:04:16.530 --> 00:04:18.440
It's actually within Earth's volume

00:04:18.440 --> 00:04:21.920
that it looks like the Moon
is rotating around the Earth.

00:04:21.920 --> 00:04:24.230
And this isn't just celestial bodies.

00:04:24.230 --> 00:04:26.460
I weigh 165 pounds.

00:04:26.460 --> 00:04:30.000
That is the force that Earth
is acting on me due to gravity,

00:04:30.000 --> 00:04:33.180
but it turns out that there's
an equal and opposite force

00:04:33.180 --> 00:04:38.180
of 165 pounds that I am
pulling on Earth with.

00:04:38.570 --> 00:04:40.050
So I will leave you there.

00:04:40.050 --> 00:04:41.200
Look around the world.

00:04:41.200 --> 00:04:42.700
This is happening everywhere.

00:04:42.700 --> 00:04:43.950
For every force,

00:04:43.950 --> 00:04:46.470
there's an equal and
opposite reaction force.

00:04:46.470 --> 00:04:47.890
But they don't cancel out

00:04:47.890 --> 00:04:50.483
because they're acting
on different bodies.


Net force

WEBVTT
Kind: captions
Language: en

00:00:00.560 --> 00:00:02.900
- [Instructor] Let's say
that we are in deep space,

00:00:02.900 --> 00:00:05.100
and there is this asteroid here,

00:00:05.100 --> 00:00:07.590
that compared to us, is stationary,

00:00:07.590 --> 00:00:10.150
or relative to us is stationary.

00:00:10.150 --> 00:00:13.480
And what we wanna do is
we wanna start to move it.

00:00:13.480 --> 00:00:18.480
So what we do is we attach
a rocket to one side,

00:00:20.530 --> 00:00:22.540
and then we ignite that rocket,

00:00:22.540 --> 00:00:26.920
and it starts to send all
this super-heated gas,

00:00:26.920 --> 00:00:30.130
all of these particles to the right.

00:00:30.130 --> 00:00:32.450
Well, what do you think that's
going to do to the asteroid?

00:00:32.450 --> 00:00:37.120
Well, it's going to push on
the asteroid in that direction.

00:00:37.120 --> 00:00:39.490
Or you could say it's
going to exert a force

00:00:39.490 --> 00:00:40.570
on that asteroid.

00:00:40.570 --> 00:00:42.980
And we could show that force like this,

00:00:42.980 --> 00:00:45.190
where the strength of that force,

00:00:45.190 --> 00:00:49.420
or the magnitude of the force,
is the length of this line.

00:00:49.420 --> 00:00:50.460
And then the direction,

00:00:50.460 --> 00:00:53.810
I will specify or show with that arrow.

00:00:53.810 --> 00:00:57.780
So fair enough, I will be
pushing towards the left.

00:00:57.780 --> 00:00:59.310
And when I push to the left,

00:00:59.310 --> 00:01:01.870
it doesn't just start to move
the asteroid to the left,

00:01:01.870 --> 00:01:05.330
it actually will accelerate
the asteroid to the left.

00:01:05.330 --> 00:01:07.790
So the longer that this rocket is running,

00:01:07.790 --> 00:01:10.980
it's going to make the
asteroid move to the left

00:01:10.980 --> 00:01:14.020
faster and faster and faster.

00:01:14.020 --> 00:01:15.800
But let's think about another example.

00:01:15.800 --> 00:01:19.360
Let's say that you and
one of your friends,

00:01:19.360 --> 00:01:21.330
you have a little bit of miscommunication,

00:01:21.330 --> 00:01:23.820
and they went and put an identical rocket

00:01:23.820 --> 00:01:26.423
on this side of the asteroid.

00:01:27.330 --> 00:01:31.120
And y'all ignited it
at the exact same time.

00:01:31.120 --> 00:01:34.370
So this one is going to
push in the other direction.

00:01:34.370 --> 00:01:35.480
What do you think is going to happen

00:01:35.480 --> 00:01:38.130
if these happened at the exact same time?

00:01:38.130 --> 00:01:40.230
Even though there's
now twice as much force

00:01:40.230 --> 00:01:41.900
being exerted on this asteroid,

00:01:41.900 --> 00:01:45.160
it's going in opposite
directions, so they zero out,

00:01:45.160 --> 00:01:47.690
and so there's zero net force.

00:01:47.690 --> 00:01:51.270
And so this asteroid won't
be accelerated at all.

00:01:51.270 --> 00:01:53.150
Now let's say that a third friend

00:01:53.150 --> 00:01:55.110
wanted to correct this situation,

00:01:55.110 --> 00:01:57.870
and this isn't necessarily the
most efficient way to do it,

00:01:57.870 --> 00:02:02.270
but what they do is they
put another identical rocket

00:02:02.270 --> 00:02:06.173
right over here, and
let's say ignite that.

00:02:07.060 --> 00:02:08.423
Now what will happen?

00:02:09.290 --> 00:02:11.600
Well, now you had the original two forces

00:02:11.600 --> 00:02:13.220
that net out to each other.

00:02:13.220 --> 00:02:15.630
But now you have this also this new force,

00:02:15.630 --> 00:02:17.880
which I will make in purple,
'cause it's a purple rocket.

00:02:17.880 --> 00:02:22.880
And so that new force,
you could draw like this,

00:02:23.630 --> 00:02:26.630
to show, all right, that
will now be the net force,

00:02:26.630 --> 00:02:28.550
'cause you have the
equivalent of two rockets

00:02:28.550 --> 00:02:29.800
going in the left direction,

00:02:29.800 --> 00:02:32.100
and one rocket going
in the right direction.

00:02:32.100 --> 00:02:34.570
Or another way we could draw that is

00:02:34.570 --> 00:02:38.270
we have two rockets going
in the left direction,

00:02:38.270 --> 00:02:41.150
so that would have a force
that looks like this.

00:02:41.150 --> 00:02:43.380
And then we have one going
in the right direction.

00:02:43.380 --> 00:02:46.000
And so if you were to net it out,

00:02:46.000 --> 00:02:48.450
this is equivalent to
just having one rocket

00:02:48.450 --> 00:02:49.930
that we originally saw.

00:02:49.930 --> 00:02:52.460
That's equivalent to just going back

00:02:52.460 --> 00:02:54.770
to what we originally saw.

00:02:54.770 --> 00:02:57.610
So the important takeaway from this video

00:02:57.610 --> 00:03:00.680
is that it's not just about
the force that's being applied.

00:03:00.680 --> 00:03:02.820
If you wanna think about how something

00:03:02.820 --> 00:03:05.400
might be accelerated,
and in what direction,

00:03:05.400 --> 00:03:07.160
you need to think about the net force,

00:03:07.160 --> 00:03:10.503
because some of the forces
could counteract each other.


Force, mass and acceleration

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:03.022
- So I have three different
asteroids over here

00:00:03.022 --> 00:00:05.500
and they have different masses.

00:00:05.500 --> 00:00:08.920
And we'll talk a lot more
about what mass means.

00:00:08.920 --> 00:00:10.618
But one way to think about it is,

00:00:10.618 --> 00:00:13.800
how much stuff there is there.

00:00:13.800 --> 00:00:15.320
There's other ways to think about it.

00:00:15.320 --> 00:00:17.920
And so let's say that this first asteroid

00:00:17.920 --> 00:00:22.880
is twice the mass of either
of these two smaller ones.

00:00:22.880 --> 00:00:26.040
And these two smaller
ones have the same mass.

00:00:26.040 --> 00:00:28.590
Now, we've attached the back of a rocket

00:00:28.590 --> 00:00:30.310
to each of these asteroids.

00:00:30.310 --> 00:00:32.729
In fact, this one over
here has two rockets,

00:00:32.729 --> 00:00:33.900
and we're going to assume

00:00:33.900 --> 00:00:35.750
that all of the rockets are equivalent

00:00:35.750 --> 00:00:37.270
and we ignite them all.

00:00:37.270 --> 00:00:42.270
And so they all exert the same
force each on the asteroid.

00:00:42.810 --> 00:00:43.740
So for example,

00:00:43.740 --> 00:00:48.740
we have a net force acting
leftward on this large asteroid.

00:00:49.070 --> 00:00:53.700
We have the same net force
acting on this smaller asteroid,

00:00:53.700 --> 00:00:55.380
also going to the left.

00:00:55.380 --> 00:00:57.329
And on this other smaller asteroid,

00:00:57.329 --> 00:01:02.150
we have two times that net
force acting to the left.

00:01:02.150 --> 00:01:04.600
So what I want you to
do is pause this video

00:01:04.600 --> 00:01:07.019
and think about which of these asteroids

00:01:07.019 --> 00:01:09.320
is going to be accelerated the most,

00:01:09.320 --> 00:01:10.720
and which of these asteroids

00:01:10.720 --> 00:01:12.803
is going to be accelerated the least.

00:01:14.520 --> 00:01:15.353
All right.

00:01:15.353 --> 00:01:20.110
So you might have an intuition
that the larger the force,

00:01:20.110 --> 00:01:22.630
the more acceleration you might see.

00:01:22.630 --> 00:01:24.390
So let me write it like this.

00:01:24.390 --> 00:01:27.830
So you might get a sense that
if you increase your force,

00:01:27.830 --> 00:01:31.430
that that's also going to
increase your acceleration.

00:01:31.430 --> 00:01:34.620
And it does turn out that
that is indeed the case.

00:01:34.620 --> 00:01:37.680
Now, the other notion that you might have

00:01:37.680 --> 00:01:40.260
is that the more of the
stuff that there is,

00:01:40.260 --> 00:01:42.450
the more mass that you have,

00:01:42.450 --> 00:01:45.300
the harder it is to accelerate it.

00:01:45.300 --> 00:01:48.070
So if you're mass is larger,

00:01:48.070 --> 00:01:51.150
than your acceleration is lower.

00:01:51.150 --> 00:01:54.860
And it turns out that these
things are all proportional.

00:01:54.860 --> 00:01:56.620
So for example,

00:01:56.620 --> 00:02:00.350
if we just compare these
two masses right over here,

00:02:00.350 --> 00:02:02.890
they have the same net force acting on it.

00:02:02.890 --> 00:02:04.370
And I keep saying net force,

00:02:04.370 --> 00:02:06.410
that means you just net
out all of the forces

00:02:06.410 --> 00:02:07.900
acting in a certain dimension.

00:02:07.900 --> 00:02:10.340
For example, if I had
another identical rocket

00:02:10.340 --> 00:02:12.970
acting in the opposite direction,

00:02:12.970 --> 00:02:15.390
they would net out, and this
asteroid right over here

00:02:15.390 --> 00:02:17.700
wouldn't be accelerated at all.

00:02:17.700 --> 00:02:19.480
But going back to our example here,

00:02:19.480 --> 00:02:22.050
we have the same net force

00:02:22.050 --> 00:02:24.410
acting on each of these asteroids.

00:02:24.410 --> 00:02:25.580
But the first asteroid

00:02:25.580 --> 00:02:28.530
has twice the mass of the second asteroid.

00:02:28.530 --> 00:02:31.030
So how do you think the
accelerations will relate?

00:02:31.930 --> 00:02:34.080
Well, as you might imagine,

00:02:34.080 --> 00:02:36.530
the acceleration on the larger asteroid

00:02:36.530 --> 00:02:39.900
is going to be half the
acceleration on this asteroid.

00:02:39.900 --> 00:02:41.330
Or another way to think about it,

00:02:41.330 --> 00:02:44.562
this asteroid is going to
have twice the acceleration

00:02:44.562 --> 00:02:46.780
as this first asteroid.

00:02:46.780 --> 00:02:50.340
And that's because it has half the mass.

00:02:50.340 --> 00:02:53.880
And one way you can relate
force, mass, and acceleration,

00:02:53.880 --> 00:02:56.161
and this is one of the
most important equations

00:02:56.161 --> 00:02:57.940
in all of physics,

00:02:57.940 --> 00:02:59.210
is that force

00:02:59.210 --> 00:03:04.210
is going to be equal to
mass times acceleration,

00:03:04.820 --> 00:03:06.560
or I could say the magnitude of the force

00:03:06.560 --> 00:03:07.890
is equal to the mass

00:03:07.890 --> 00:03:10.430
times the magnitude of the acceleration.

00:03:10.430 --> 00:03:12.870
So notice, in this
example right over here,

00:03:12.870 --> 00:03:16.270
our forces are the same, but
the masses are different.

00:03:16.270 --> 00:03:18.900
If I have half the mass
as I have over here,

00:03:18.900 --> 00:03:21.480
I'm going to have twice the acceleration.

00:03:21.480 --> 00:03:23.340
And that might make intuitive sense

00:03:23.340 --> 00:03:25.490
if you've ever tried
to apply the same force

00:03:25.490 --> 00:03:27.250
to something that has a small mass

00:03:27.250 --> 00:03:29.960
versus something that has a large mass.

00:03:29.960 --> 00:03:33.900
Now, if we compare these two asteroids,

00:03:33.900 --> 00:03:36.690
they have the same mass here,

00:03:36.690 --> 00:03:38.930
but the force here, the net force,

00:03:38.930 --> 00:03:41.680
acting in that left direction, is double.

00:03:41.680 --> 00:03:44.240
So if you double the force,
don't change the mass.

00:03:44.240 --> 00:03:45.922
Well, then you're going to
have twice the acceleration.

00:03:45.922 --> 00:03:49.680
So this is going to have twice
the acceleration of this one,

00:03:49.680 --> 00:03:50.513
and this one's

00:03:50.513 --> 00:03:52.930
going to have twice the
acceleration of that one.

00:03:52.930 --> 00:03:54.215
But the important thing to realize

00:03:54.215 --> 00:03:58.403
is how force, mass, and
acceleration are connected.