2015 AP Physics 1 free response 4

WEBVTT
Kind: captions
Language: en

00:00:00.062 --> 00:00:02.626
- [Voiceover] Two identical
spheres are released from

00:00:02.626 --> 00:00:06.725
a device at time equals
zero, from the same height H,

00:00:06.725 --> 00:00:09.267
as shown above, or T
equals zero I should say.

00:00:09.267 --> 00:00:12.239
Sphere A has no initial velocity
and falls straight down.

00:00:12.239 --> 00:00:15.838
Sphere B is given an initial
horizontal velocity of

00:00:15.838 --> 00:00:19.739
magnitude V sub zero, and
travels a horizontal distance D

00:00:19.739 --> 00:00:21.783
before it reaches the ground.

00:00:21.783 --> 00:00:26.148
The spheres reach the ground
at the same time T sub F,

00:00:26.148 --> 00:00:28.088
even though sphere B has more distance

00:00:28.088 --> 00:00:29.795
to cover before landing.

00:00:29.795 --> 00:00:32.545
Air resistance is negligible.

00:00:32.545 --> 00:00:35.669
The dots below represent spheres A and B.

00:00:35.669 --> 00:00:39.209
Draw a free-body diagram
showing and labeling the forces,

00:00:39.209 --> 00:00:42.088
not components, exerted on each sphere

00:00:42.088 --> 00:00:44.551
at time T sub F over two.

00:00:44.551 --> 00:00:46.129
So we can see our spheres here,

00:00:46.129 --> 00:00:49.148
when I guess this little
this thing releases,

00:00:49.148 --> 00:00:50.947
sphere A goes straight down.

00:00:50.947 --> 00:00:55.231
Sphere B, it it will
go, well it's vertical,

00:00:55.231 --> 00:00:57.669
and the vertical direction,
it'll go down just the same way.

00:00:57.669 --> 00:00:59.706
It'll be accelerated in just
the same way as sphere A,

00:00:59.706 --> 00:01:03.165
but it has some horizontal
velocity that makes it move out

00:01:03.165 --> 00:01:07.183
and hit the ground D to the right.

00:01:07.183 --> 00:01:09.865
And when it hits the
ground, that's T sub F.

00:01:09.865 --> 00:01:11.967
When they're up here, that's
right when they're released,

00:01:11.967 --> 00:01:16.111
it's T equals zero, and then
this is at T equals T sub F.

00:01:16.111 --> 00:01:20.849
And they say a free-body
diagram at T sub F over two.

00:01:20.849 --> 00:01:24.912
So this is while both
of them are in flight.

00:01:24.912 --> 00:01:28.325
So while both of them are in flight,

00:01:28.325 --> 00:01:31.413
the only force acting on each of them,

00:01:31.413 --> 00:01:33.248
is just going to be the force of gravity.

00:01:33.248 --> 00:01:35.186
And since the spheres are identical,

00:01:35.186 --> 00:01:37.091
the force of that gravity
is going to be identical.

00:01:37.091 --> 00:01:41.072
They have the same mass, so let me draw.

00:01:41.072 --> 00:01:43.290
So that right over there
is the force of gravity

00:01:43.290 --> 00:01:47.477
on sphere A, and that is
the force of gravity on

00:01:47.477 --> 00:01:49.452
sphere B.

00:01:49.452 --> 00:01:53.281
And so we could write, force of gravity,

00:01:53.281 --> 00:01:56.630
force of,

00:01:56.630 --> 00:01:59.110
force of gravity.

00:02:00.140 --> 00:02:03.602
And if we want, we could,
we could say the magnitude

00:02:03.602 --> 00:02:06.981
is F sub G, if we want. F sub G.

00:02:06.981 --> 00:02:11.981
Or we could label it as M
times the gravitational field.

00:02:12.020 --> 00:02:13.761
So this is equal to,

00:02:13.761 --> 00:02:17.259
is equal to M times the
gravitational field.

00:02:17.259 --> 00:02:19.441
And that's it, while they're mid flight,

00:02:19.441 --> 00:02:20.707
the only force acting on them,

00:02:20.707 --> 00:02:22.785
we're assuming air
resistance is negligible,

00:02:22.785 --> 00:02:25.060
is the force of gravity's
going to be the same because

00:02:25.060 --> 00:02:28.321
they have the same mass,
they're identical spheres.

00:02:28.321 --> 00:02:31.400
Alright, let's tackle
the next part of this.

00:02:31.400 --> 00:02:35.102
On the axes below,
sketch and label a graph

00:02:35.102 --> 00:02:39.318
of the horizontal components
of the velocity of sphere A

00:02:39.318 --> 00:02:42.335
and of sphere B as a function of time.

00:02:42.335 --> 00:02:43.659
Alright, I'll do sphere A first.

00:02:43.659 --> 00:02:45.539
This is pretty straight forward.

00:02:45.539 --> 00:02:49.440
Sphere A, if you will
remember, let's go up here.

00:02:49.440 --> 00:02:54.096
Sphere A has no horizontal
velocity the entire time

00:02:54.096 --> 00:02:55.561
we're talking about it, it only,

00:02:55.561 --> 00:02:58.113
it's only going to be accelerated
in the vertical direction.

00:02:58.113 --> 00:03:00.156
It's going to be accelerated downwards.

00:03:00.156 --> 00:03:03.547
So sphere A has no horizontal velocity,

00:03:03.547 --> 00:03:06.530
so I will draw a line like this.

00:03:06.530 --> 00:03:11.530
So sphere A has no horizontal
velocity the entire time.

00:03:13.540 --> 00:03:17.478
Now sphere, sphere B,

00:03:17.478 --> 00:03:20.498
sphere B is going to be

00:03:20.498 --> 00:03:23.660
a little bit more interesting,
slightly more interesting.

00:03:23.660 --> 00:03:27.027
It's velocity, they tell us,
that it's initial velocity is

00:03:27.027 --> 00:03:30.928
V sub zero, it's initial
horizontal velocity I should say,

00:03:30.928 --> 00:03:34.666
has a magnitude of V sub zero.

00:03:34.666 --> 00:03:37.023
And since air resistance is negligible,

00:03:37.023 --> 00:03:39.683
it's gonna continue going
to the right at V sub zero

00:03:39.683 --> 00:03:42.201
until it hits the ground.

00:03:42.201 --> 00:03:47.201
So, so sphere B, this is,

00:03:47.380 --> 00:03:49.817
and I'm just gonna pick
one of these as V sub zero.

00:03:49.817 --> 00:03:53.602
Let's say that this right
over here is V sub zero.

00:03:53.602 --> 00:03:56.680
That's the magnitude of
it's horizontal velocity.

00:03:56.680 --> 00:04:00.324
Well sphere B is going
to be at that velocity,

00:04:00.324 --> 00:04:03.378
actually let me just make
it a little bit clearer.

00:04:03.378 --> 00:04:08.271
It's gonna be at that
velocity until, until V F.

00:04:08.271 --> 00:04:11.959
So if we say this right
over here, or not V F,

00:04:11.959 --> 00:04:15.223
until the final time, until T F.

00:04:15.223 --> 00:04:16.860
So this is T equals zero to T F.

00:04:16.860 --> 00:04:18.636
The entire time while the ball's in the,

00:04:18.636 --> 00:04:20.787
while that sphere is in the air,

00:04:20.787 --> 00:04:22.981
it's going to have the
horizontal component

00:04:22.981 --> 00:04:25.605
of its velocity is just
going to be constant.

00:04:25.605 --> 00:04:26.928
It's not going to be
slowed down by anything

00:04:26.928 --> 00:04:28.960
because we're assuming air
resistance is negligible.

00:04:28.960 --> 00:04:31.085
And then right when it hits the ground,

00:04:31.085 --> 00:04:32.629
it essentially, if you
think about the force

00:04:32.629 --> 00:04:35.288
that is stopping it is
essentially friction,

00:04:35.288 --> 00:04:40.288
but then it very quickly
goes down to a velocity of a

00:04:41.161 --> 00:04:42.938
a magnitude of velocity,

00:04:42.938 --> 00:04:46.606
of horizontal magnitude velocity of zero.

00:04:46.606 --> 00:04:51.606
Alright, alright now let's
tackle the last part of this.

00:04:52.263 --> 00:04:53.947
Now you could label this if you want,

00:04:53.947 --> 00:04:56.037
this is, let me actually let me label it,

00:04:56.037 --> 00:05:01.037
this is B, sphere B, and this is sphere,

00:05:01.842 --> 00:05:04.663
that is sphere A right over there.

00:05:04.663 --> 00:05:06.439
In sphere B if you want, you could show,

00:05:06.439 --> 00:05:08.297
it would overwrite sphere A, so your B

00:05:08.297 --> 00:05:09.760
would be zero after that.

00:05:09.760 --> 00:05:12.987
It's not continuing to
move on to the right,

00:05:12.987 --> 00:05:15.509
or at least they don't tell
us anything about, about that.

00:05:15.509 --> 00:05:19.190
Finally, in a clear, coherent, paragl

00:05:19.190 --> 00:05:20.153
(laughs)

00:05:20.153 --> 00:05:23.229
clear, coherent,
paragraph-length response,

00:05:23.229 --> 00:05:26.585
explain why the spheres reach
the ground at the same time

00:05:26.585 --> 00:05:29.568
even though they travel
different distances.

00:05:29.568 --> 00:05:34.568
Include references in your
answers to parts A and B.

00:05:34.747 --> 00:05:36.128
Alright, so let me think about it.

00:05:36.128 --> 00:05:37.475
I'll try to write a clear, coherent,

00:05:37.475 --> 00:05:39.936
paragraph-length response.

00:05:39.936 --> 00:05:44.936
So I'll say, the entire time the,

00:05:45.113 --> 00:05:46.951
or let me say from,

00:05:46.951 --> 00:05:51.395
from T equals zero...

00:05:51.395 --> 00:05:55.045
to T equals T sub F,

00:05:55.045 --> 00:05:58.665
the only force

00:06:00.603 --> 00:06:05.601
acting on the spheres

00:06:08.695 --> 00:06:11.744
is the downward force of gravity.

00:06:11.744 --> 00:06:16.744
Is the downward force,

00:06:18.723 --> 00:06:23.723
force of (mumbles) of gravity.

00:06:25.143 --> 00:06:28.545
At T equals zero,

00:06:28.545 --> 00:06:32.484
at T equals zero, they both,

00:06:32.484 --> 00:06:37.484
they both have zero vertical velocity or

00:06:38.262 --> 00:06:41.385
the magnitude of the velocity
in the vertical direction

00:06:41.385 --> 00:06:42.605
is zero for both of em.

00:06:42.605 --> 00:06:44.311
Let me write it that way.

00:06:44.311 --> 00:06:49.140
The, the magnitude

00:06:50.570 --> 00:06:53.879
of both of their velocities,

00:06:53.879 --> 00:06:58.879
both of their velocities,

00:07:01.322 --> 00:07:05.245
velocities,

00:07:05.245 --> 00:07:09.613
in the vertical

00:07:09.613 --> 00:07:11.733
direction

00:07:11.733 --> 00:07:15.033
(writes sentence)

00:07:15.033 --> 00:07:17.563
is zero.

00:07:17.563 --> 00:07:22.563
After T equals zero,

00:07:22.855 --> 00:07:25.795
they are accelerated,

00:07:25.795 --> 00:07:28.484
they are accelerated at the same rate.

00:07:28.484 --> 00:07:31.598
Accelerated

00:07:31.598 --> 00:07:33.098
(writing)

00:07:33.098 --> 00:07:36.778
at the same,

00:07:36.778 --> 00:07:40.856
they're accelerated at the same rate.

00:07:40.856 --> 00:07:43.967
so their vertical component of velocity,

00:07:43.967 --> 00:07:45.207
their vertical

00:07:45.207 --> 00:07:48.037
(writing)

00:07:48.037 --> 00:07:53.037
component, components of velocity,

00:07:53.567 --> 00:07:55.507
velocity are always the same.

00:07:55.507 --> 00:08:00.283
Of velocity are always

00:08:00.283 --> 00:08:02.423
(writing)

00:08:02.423 --> 00:08:03.893
the same.

00:08:03.893 --> 00:08:06.792
And they have the same
vertical distance to cover,

00:08:06.792 --> 00:08:11.618
and they have the same,

00:08:11.618 --> 00:08:15.028
(writing)

00:08:15.028 --> 00:08:17.518
the same vertical distance to cover.

00:08:19.598 --> 00:08:24.258
Vertical distance to cover.

00:08:25.948 --> 00:08:28.653
So they hit the ground at the same time.

00:08:28.653 --> 00:08:29.814
Let me make sure that makes sense.

00:08:29.814 --> 00:08:32.414
After T equals zero, they are
accelerated at the same rate,

00:08:32.414 --> 00:08:34.411
so their vertical
components of velocity are

00:08:34.411 --> 00:08:36.675
always the same.

00:08:36.675 --> 00:08:40.808
Let me, actually let me,
let me write this this way.

00:08:40.808 --> 00:08:45.808
Since they have the same, since,

00:08:46.950 --> 00:08:48.484
actually let me,

00:08:48.484 --> 00:08:53.484
since they have the same

00:08:57.152 --> 00:08:59.132
vertical distance to cover,

00:08:59.132 --> 00:09:03.659
vertical distance to cover,

00:09:03.659 --> 00:09:06.019
(writing)

00:09:06.019 --> 00:09:08.941
they will hit the ground at the same time.

00:09:08.941 --> 00:09:13.401
They will hit the ground

00:09:13.401 --> 00:09:15.631
(writes sentence)

00:09:15.631 --> 00:09:19.061
at the same time.

00:09:19.901 --> 00:09:21.451
Same time.

00:09:22.731 --> 00:09:25.546
They do have different
horizontal velocities,

00:09:25.546 --> 00:09:29.387
but that does not affect their,
that does effect the time

00:09:29.387 --> 00:09:34.387
their velocities or the distance
in the vertical direction.

00:09:35.774 --> 00:09:40.613
They have different horizontal,

00:09:40.613 --> 00:09:44.953
(writes sentence)

00:09:44.953 --> 00:09:48.183
horizontal velocities,

00:09:50.263 --> 00:09:53.739
but that

00:09:53.739 --> 00:09:55.159
(writes sentence)

00:09:55.159 --> 00:09:59.039
does not effect

00:09:59.039 --> 00:10:01.339
the time in which they,

00:10:01.339 --> 00:10:02.756
they cover the same vertical distance,

00:10:02.756 --> 00:10:07.473
effect the time in which

00:10:07.473 --> 00:10:10.143
(writes sentence)

00:10:10.143 --> 00:10:15.143
they cover the same

00:10:16.333 --> 00:10:19.694
vertical distance.

00:10:19.694 --> 00:10:22.793
And you could write
something to that effect,

00:10:22.793 --> 00:10:26.056
and you could also write
that yes, if you were to add

00:10:26.056 --> 00:10:28.818
the components of spheres Bs velocities,

00:10:28.818 --> 00:10:31.257
it would actually have a larger velocity

00:10:31.257 --> 00:10:32.395
if you were to add the components.

00:10:32.395 --> 00:10:33.579
If you're not thinking you needed

00:10:33.579 --> 00:10:34.554
the horizontal or the vertical direction,

00:10:34.554 --> 00:10:38.814
and so it does indeed cover
more distance and space

00:10:38.814 --> 00:10:40.197
over the same amount of time.

00:10:40.197 --> 00:10:42.471
But if you think about it just
in the vertical direction,

00:10:42.471 --> 00:10:44.655
it's covering the same
distance, in the same time,

00:10:44.655 --> 00:10:46.872
at any given point in time
in the vertical direction.

00:10:46.872 --> 00:10:48.218
It actually has the same velocity

00:10:48.218 --> 00:10:50.379
It's being accelerated in the same way

00:10:50.379 --> 00:10:55.301
that starts off at, of
magnitude of velocity of zero.


2015 AP Physics 1 free response 2 c and d

WEBVTT
Kind: captions
Language: en

00:00:00.177 --> 00:00:02.682
- [Voiceover] Let's now tackle
the rest of this problem.

00:00:02.682 --> 00:00:05.315
So they say a light-bulb is non-ohmic

00:00:05.315 --> 00:00:08.501
if its resistance changes
as a function of current.

00:00:08.501 --> 00:00:11.297
Your setup from part a, which
we have right over here,

00:00:11.297 --> 00:00:13.865
is to be used or modified
to determine whether

00:00:13.865 --> 00:00:16.336
the light-bulb is non-ohmic.

00:00:16.336 --> 00:00:20.954
How, if at all, does the
setup need to be modified?

00:00:20.954 --> 00:00:22.879
Well, we want to be able
to change the current

00:00:22.879 --> 00:00:24.147
that's going through the light-bulb

00:00:24.147 --> 00:00:27.430
and then see how the resistance changes,

00:00:27.430 --> 00:00:29.931
so what we could do, we
could always measure it

00:00:29.931 --> 00:00:31.031
with the setup right over here.

00:00:31.031 --> 00:00:33.181
It's very easy to measure the resistance.

00:00:33.181 --> 00:00:35.814
So we can measure resistance by,

00:00:35.814 --> 00:00:37.797
we know that V is equal to,

00:00:37.797 --> 00:00:40.137
we know that voltage is equal
to current times resistance

00:00:40.137 --> 00:00:43.908
or that resistance is equal
to voltage divided by current.

00:00:43.908 --> 00:00:45.566
So, if we know the current

00:00:45.566 --> 00:00:47.419
going through a light-bulb

00:00:47.419 --> 00:00:49.012
and we know the voltage across it,

00:00:49.012 --> 00:00:51.255
we can always figure out the
resistance of the light-bulb.

00:00:51.255 --> 00:00:53.206
So our set-up right here
is actually very good

00:00:53.206 --> 00:00:54.919
for figuring out the resistance

00:00:54.919 --> 00:00:57.125
of the actual light-bulb.

00:00:57.125 --> 00:00:58.783
And so we need to change the current

00:00:58.783 --> 00:01:00.668
and see how the resistance changes.

00:01:00.668 --> 00:01:01.681
And there's a couple of ways

00:01:01.681 --> 00:01:03.116
that we could change the current.

00:01:03.116 --> 00:01:05.489
We could change some
other resistance here,

00:01:05.489 --> 00:01:08.187
but lucky for us we have
a variable power source,

00:01:08.187 --> 00:01:10.755
so we can change the voltage,

00:01:10.755 --> 00:01:13.421
we can change the voltage
across the entire circuit,

00:01:13.421 --> 00:01:14.564
and by changing the voltage

00:01:14.564 --> 00:01:16.096
across the entire circuit,

00:01:16.096 --> 00:01:18.339
well that's going to change
the current that is going.

00:01:18.339 --> 00:01:19.172
This is a simple circuit.

00:01:19.172 --> 00:01:20.465
It's just going in series.

00:01:20.465 --> 00:01:22.871
The current is just doing like that,

00:01:22.871 --> 00:01:25.764
and so if we change the current
through the entire circuit,

00:01:25.764 --> 00:01:28.072
we would be changing the
current through the light-bulb,

00:01:28.072 --> 00:01:30.478
and then we could just measure V and I

00:01:30.478 --> 00:01:33.895
and figure out whether R changes as well.

00:01:35.354 --> 00:01:38.252
So, how, if all, does the
setup need to be modified?

00:01:38.252 --> 00:01:41.015
I could say it doesn't
need to be modified.

00:01:41.015 --> 00:01:43.182
Doesn't need modification.

00:01:45.761 --> 00:01:46.845
Modification.

00:01:48.722 --> 00:01:49.992
All right.

00:01:49.992 --> 00:01:51.130
And then the next section,

00:01:51.130 --> 00:01:52.853
what additional data, if any,

00:01:52.853 --> 00:01:55.551
would need to be collected.

00:01:55.551 --> 00:01:57.696
Well, I would want to,

00:01:57.696 --> 00:01:59.679
I would want to measure.

00:01:59.679 --> 00:02:01.054
Let me even do this part down.

00:02:01.054 --> 00:02:03.102
Let me do it, part c, over here.

00:02:03.102 --> 00:02:04.018
So, part c.

00:02:05.995 --> 00:02:09.578
So, part one of part
c was no modification.

00:02:12.250 --> 00:02:13.584
No modification.

00:02:16.964 --> 00:02:18.752
No mod, I can explain that.

00:02:18.752 --> 00:02:20.800
I can change resistance.

00:02:20.800 --> 00:02:21.883
I can change,

00:02:23.925 --> 00:02:25.591
I can change current

00:02:29.266 --> 00:02:31.769
by varying power source.

00:02:31.769 --> 00:02:33.769
By varying power source.

00:02:36.190 --> 00:02:37.273
Power source.

00:02:38.693 --> 00:02:40.676
I can measure,

00:02:40.676 --> 00:02:42.676
I can measure resistance

00:02:48.487 --> 00:02:49.320
of bulb

00:02:51.743 --> 00:02:53.659
by voltage across bulb,

00:02:58.212 --> 00:02:59.212
across bulb,

00:03:00.715 --> 00:03:01.631
divided by,

00:03:02.730 --> 00:03:04.230
divided by current

00:03:06.696 --> 00:03:08.780
entering or exiting bulb.

00:03:10.894 --> 00:03:12.780
And I know the first part of this test

00:03:12.780 --> 00:03:13.613
we were saying, hey,

00:03:13.613 --> 00:03:16.712
do we have the same number
of electrons per second

00:03:16.712 --> 00:03:18.845
entering the bulb or exiting the bulb,

00:03:18.845 --> 00:03:20.077
but from what we know of currents,

00:03:20.077 --> 00:03:21.731
we know that the current
is going to be the same

00:03:21.731 --> 00:03:22.618
on either side.

00:03:22.618 --> 00:03:25.386
You have a voltage drop,
but the current is the same.

00:03:25.386 --> 00:03:26.816
And so entering or exiting,

00:03:26.816 --> 00:03:28.084
you're going to get the same value,

00:03:28.084 --> 00:03:29.872
and even if you didn't
know what I just told you,

00:03:29.872 --> 00:03:31.767
the first part of the experiment
would have told you that,

00:03:31.767 --> 00:03:32.681
that your current is the same

00:03:32.681 --> 00:03:34.696
entering and exiting the bulb.

00:03:34.696 --> 00:03:36.054
And so, this right over here

00:03:36.054 --> 00:03:40.138
is going to be equal to
the resistance of the bulb.

00:03:40.138 --> 00:03:45.052
So I can change the current
by varying the power source,

00:03:45.052 --> 00:03:48.563
and then I can see what value I get for R

00:03:48.563 --> 00:03:50.095
by measuring these things

00:03:50.095 --> 00:03:52.538
as I change that overall current for my,

00:03:52.538 --> 00:03:53.621
for my setup.

00:03:56.351 --> 00:03:57.196
Let's see b.

00:03:57.196 --> 00:04:00.154
What additional data, if any,
would need to be collected?

00:04:00.154 --> 00:04:01.237
So, part two.

00:04:03.322 --> 00:04:05.263
Part two here.

00:04:05.263 --> 00:04:07.697
I'd need to collect,

00:04:07.697 --> 00:04:08.947
collect voltage

00:04:13.029 --> 00:04:13.862
across,

00:04:15.207 --> 00:04:16.207
across bulb

00:04:17.287 --> 00:04:18.871
as current changes,

00:04:21.968 --> 00:04:24.961
or it'd be as I change
my variable power source.

00:04:24.961 --> 00:04:26.961
As I vary power source,

00:04:31.142 --> 00:04:32.813
power source,

00:04:32.813 --> 00:04:34.796
and I would also have to collect,

00:04:34.796 --> 00:04:35.772
maybe I'll write it this way,

00:04:35.772 --> 00:04:38.276
I'll collect that, voltage.

00:04:38.276 --> 00:04:39.109
I'll write it like that.

00:04:39.109 --> 00:04:41.643
Voltage across bulb as
I vary power source,

00:04:41.643 --> 00:04:43.643
and I could say current,

00:04:44.914 --> 00:04:46.330
current entering,

00:04:48.774 --> 00:04:49.774
exiting bulb

00:04:54.496 --> 00:04:56.412
as I vary power source.

00:05:00.101 --> 00:05:01.184
Power source.

00:05:02.121 --> 00:05:04.136
So I'd have to measure these things

00:05:04.136 --> 00:05:06.835
and then I'll be able to
figure out if my R changes.

00:05:06.835 --> 00:05:09.955
All right, let's tackle part d now.

00:05:09.955 --> 00:05:11.191
How would you analyze the data

00:05:11.191 --> 00:05:13.893
to determine whether the bulb I non-ohmic?

00:05:13.893 --> 00:05:15.746
Include a discussion of
how the uncertainties

00:05:15.746 --> 00:05:19.680
in the voltmeters and ammeters
would affect your argument

00:05:19.680 --> 00:05:23.938
for concluding whether the
resistance is non-ohmic.

00:05:23.938 --> 00:05:27.839
Well, I explained it a
little bit, well, up here,

00:05:27.839 --> 00:05:29.790
but I'll write it again.

00:05:29.790 --> 00:05:32.271
d, so, let me read that one more time.

00:05:32.271 --> 00:05:34.519
How would you analyze the data?

00:05:34.519 --> 00:05:36.435
So, what I would do is,

00:05:37.834 --> 00:05:39.251
so, power source,

00:05:41.768 --> 00:05:43.986
I would make one voltage for,

00:05:43.986 --> 00:05:46.152
I would vary power source,

00:05:49.867 --> 00:05:51.867
source to first voltage,

00:05:55.143 --> 00:05:55.976
voltage,

00:05:58.486 --> 00:05:59.486
and measure,

00:06:02.050 --> 00:06:04.715
measure what is described above,

00:06:04.715 --> 00:06:06.215
and measure above.

00:06:07.596 --> 00:06:09.013
Use to calculate,

00:06:13.187 --> 00:06:16.341
use to calculate the resistance,

00:06:16.341 --> 00:06:17.924
resistance of bulb.

00:06:21.965 --> 00:06:23.215
And then, vary

00:06:26.105 --> 00:06:27.105
power source

00:06:30.161 --> 00:06:31.661
to second voltage,

00:06:34.159 --> 00:06:36.159
voltage, and once again,

00:06:37.410 --> 00:06:38.577
measure above,

00:06:39.783 --> 00:06:40.950
measure above.

00:06:43.219 --> 00:06:45.757
See if resistance changes,

00:06:45.757 --> 00:06:47.257
see if resistance

00:06:51.265 --> 00:06:52.098
changes.

00:06:55.288 --> 00:06:57.336
So, we could say,

00:06:57.336 --> 00:06:58.836
if V across bulb,

00:07:01.636 --> 00:07:02.636
across bulb,

00:07:05.454 --> 00:07:08.012
over, let me actually, let
me just write it this way.

00:07:08.012 --> 00:07:09.578
All the Vs are cross above with.

00:07:09.578 --> 00:07:11.749
V_one over the current_one,

00:07:11.749 --> 00:07:13.481
so this is the voltage across the bulb

00:07:13.481 --> 00:07:15.594
over the current across the bulb.

00:07:15.594 --> 00:07:19.592
If this is equal to, after I change it,

00:07:19.592 --> 00:07:21.315
the voltage across the bulb

00:07:21.315 --> 00:07:24.696
divided by the current across the bulb.

00:07:24.696 --> 00:07:26.419
If this over here,

00:07:26.419 --> 00:07:29.540
if these are equal,
then I am dealing with,

00:07:29.540 --> 00:07:31.783
the resistance has not changed,

00:07:31.783 --> 00:07:33.449
and so we could say,

00:07:34.646 --> 00:07:36.208
well non-ohmic,

00:07:36.208 --> 00:07:39.348
non-ohmic is when the
resistance does change.

00:07:39.348 --> 00:07:40.569
If resistance, so we could say,

00:07:40.569 --> 00:07:43.439
non-non-ohmic, or I could say

00:07:43.439 --> 00:07:44.605
not non-ohmic,

00:07:47.372 --> 00:07:48.205
non-ohmic,

00:07:50.981 --> 00:07:54.814
otherwise if V_one over
I_one does not equal,

00:07:55.954 --> 00:07:57.714
let me write that, I wrote V_i.

00:07:57.714 --> 00:07:59.381
Let me write V_one,

00:08:00.431 --> 00:08:03.844
V_one over I_one does not equal

00:08:03.844 --> 00:08:07.177
V_two over I_two, then we are non-ohmic.

00:08:10.866 --> 00:08:12.686
And we have to be careful,

00:08:12.686 --> 00:08:17.302
because the sensitivities of
our voltmeters and ammeters

00:08:17.302 --> 00:08:19.090
might give us an aberrant result.

00:08:19.090 --> 00:08:20.976
For example, we might be non-ohmic

00:08:20.976 --> 00:08:23.316
but because of how we're
rounding in the measurement,

00:08:23.316 --> 00:08:25.104
it actually might give us a change,

00:08:25.104 --> 00:08:28.011
especially because our ammeter
is more sensitive here,

00:08:28.011 --> 00:08:31.002
and on the other hand, we
might be a situation where

00:08:31.002 --> 00:08:34.123
we are non-ohmic, but it doesn't,

00:08:34.123 --> 00:08:36.041
you don't register the
change because it hasn't

00:08:36.041 --> 00:08:37.406
been large enough to be registered

00:08:37.406 --> 00:08:40.104
by the sensitivity of your instruments.

00:08:40.104 --> 00:08:42.021
So, need to be careful,

00:08:43.392 --> 00:08:44.642
careful about,

00:08:46.778 --> 00:08:49.606
about instrument sensitivity.

00:08:49.606 --> 00:08:51.523
Instrument sensitivity.

00:08:53.576 --> 00:08:54.576
Sensitivity.

00:08:57.055 --> 00:08:58.138
The rounding,

00:08:59.948 --> 00:09:01.615
rounding to nearest,

00:09:05.539 --> 00:09:06.956
nearest 0.1 volt

00:09:08.312 --> 00:09:09.228
or 0.01 amp

00:09:12.505 --> 00:09:13.839
could result in,

00:09:17.837 --> 00:09:19.003
in difference

00:09:22.100 --> 00:09:23.016
in V over I

00:09:24.700 --> 00:09:26.534
when there isn't any,

00:09:27.431 --> 00:09:28.764
there isn't any,

00:09:31.202 --> 00:09:32.369
or vice versa.

00:09:33.582 --> 00:09:35.604
It might not result in a difference,

00:09:35.604 --> 00:09:36.840
but there actually might be a change

00:09:36.840 --> 00:09:38.400
but it's not big enough to be measured.

00:09:38.400 --> 00:09:39.567
Or vice versa.

00:09:40.838 --> 00:09:42.008
So you always have to be careful

00:09:42.008 --> 00:09:43.926
when you're doing any
experiment in any science

00:09:43.926 --> 00:09:46.755
about how sensitive your
actual instruments are

00:09:46.755 --> 00:09:48.120
and what the rounding might do

00:09:48.120 --> 00:09:50.037
for these calculations.


2015 AP Physics I free response 2 a and b

WEBVTT
Kind: captions
Language: en

00:00:00.547 --> 00:00:02.301
- [Voiceover] Some students
want to know what gets

00:00:02.301 --> 00:00:05.698
used up in an incandescent
lightbulb when it is in series

00:00:05.698 --> 00:00:09.575
with a resistor: current, energy, or both.

00:00:09.575 --> 00:00:12.282
They come up with the
following two questions.

00:00:12.282 --> 00:00:15.740
In one second, do fewer
electrons leave the bulb

00:00:15.740 --> 00:00:17.726
than enter the bulb?

00:00:17.726 --> 00:00:21.023
Does the electric potential
energy of electrons change

00:00:21.023 --> 00:00:23.206
while inside the bulb?

00:00:23.206 --> 00:00:25.439
So this first question is
really a measure of current

00:00:25.439 --> 00:00:27.622
because current is how
much charge per second

00:00:27.622 --> 00:00:31.441
is going through a particular
part of our circuit,

00:00:31.441 --> 00:00:33.984
so charge per second.

00:00:33.984 --> 00:00:35.969
You could also think of
electrons per second.

00:00:35.969 --> 00:00:37.746
So current's going to
measure the first one,

00:00:37.746 --> 00:00:39.881
and the electric potential
energy of the electrons,

00:00:39.881 --> 00:00:41.519
well that's going to be voltage.

00:00:41.519 --> 00:00:42.598
Does the voltage change?

00:00:42.598 --> 00:00:44.143
Is there a voltage drop when we go

00:00:44.143 --> 00:00:46.383
from one side of the bulb to another?

00:00:46.383 --> 00:00:48.508
The students have an
adjustable power source,

00:00:48.508 --> 00:00:50.946
insulated wire, lightbulbs, resistors,

00:00:50.946 --> 00:00:52.965
switches, voltmeters, ammeters,

00:00:52.965 --> 00:00:55.021
and other standard lab equipment.

00:00:55.021 --> 00:00:57.401
Assume the power supply and voltmeters

00:00:57.401 --> 00:01:01.140
are marked in tenths of a volt increments

00:01:01.140 --> 00:01:04.065
and ammeters are marked in hundredths

00:01:04.065 --> 00:01:08.024
of an amp increments.

00:01:08.024 --> 00:01:09.928
Describe an experimental procedure

00:01:09.928 --> 00:01:11.983
that could be used to answer questions one

00:01:11.983 --> 00:01:13.168
and two above.

00:01:13.168 --> 00:01:15.362
In your description,
state the measurements

00:01:15.362 --> 00:01:17.404
you would make and how you would use

00:01:17.404 --> 00:01:19.008
the equipment to make them.

00:01:19.008 --> 00:01:23.830
Include a neat, labeled
diagram of your setup.

00:01:23.830 --> 00:01:28.050
All right, so let's do
part A right over here.

00:01:30.621 --> 00:01:32.729
So part A, so we're going to have

00:01:32.729 --> 00:01:33.808
a power source.

00:01:33.808 --> 00:01:35.828
I think they said it was
a variable power source.

00:01:35.828 --> 00:01:37.187
Did they say that?

00:01:37.187 --> 00:01:41.324
Yeah, the students have an
adjustable power source,

00:01:41.324 --> 00:01:43.158
insulated wire, lightbulbs, resistors,

00:01:43.158 --> 00:01:44.099
switches, and voltmeters.

00:01:44.099 --> 00:01:46.003
They want to know an incandescent bulb,

00:01:46.003 --> 00:01:49.637
lightbulb, when it is in
series with a resistor.

00:01:49.637 --> 00:01:51.123
So an incandescent lightbulb

00:01:51.123 --> 00:01:53.758
when it is in series with a resistor.

00:01:53.758 --> 00:01:56.800
So let's make an attempt at drawing this.

00:01:56.800 --> 00:01:59.285
So just the circuit, the circuit only,

00:01:59.285 --> 00:02:00.608
that they care about,

00:02:00.608 --> 00:02:02.721
we could do our adjustable power source.

00:02:02.721 --> 00:02:04.579
So let me draw that.

00:02:04.579 --> 00:02:07.580
So you could do that like this.

00:02:08.552 --> 00:02:10.723
I'll do a couple here.

00:02:10.723 --> 00:02:13.567
So like this.

00:02:13.567 --> 00:02:14.647
So that would be a power source

00:02:14.647 --> 00:02:16.191
where this is the positive end,

00:02:16.191 --> 00:02:17.712
this is the negative end,

00:02:17.712 --> 00:02:21.311
and then let me make my circuit

00:02:21.311 --> 00:02:22.530
before any measurement tools.

00:02:22.530 --> 00:02:24.562
Then we'll add the measurement tools.

00:02:24.562 --> 00:02:25.491
So I'm going to make it in series

00:02:25.491 --> 00:02:27.429
with a resistor.

00:02:27.429 --> 00:02:30.983
So let's put a resistor here

00:02:30.983 --> 00:02:33.002
of some resistance.

00:02:33.002 --> 00:02:36.683
Then let's keep going with our circuit.

00:02:36.683 --> 00:02:37.611
So we're going to keep going

00:02:37.611 --> 00:02:38.923
with our circuit.

00:02:38.923 --> 00:02:40.746
And now let's put our
incandescent lightbulb.

00:02:40.746 --> 00:02:42.232
The symbol for an incandescent lightbulb,

00:02:42.232 --> 00:02:43.161
there's actually several,

00:02:43.161 --> 00:02:46.639
I'll use one where I do a little bump here

00:02:46.639 --> 00:02:47.979
and then I'll continue

00:02:47.979 --> 00:02:50.573
and I'm actually gonna put
a circle around that bump.

00:02:50.573 --> 00:02:54.985
So it would be just like I'm almost there.

00:02:54.985 --> 00:02:55.809
I'm trying to do it neatly

00:02:55.809 --> 00:02:56.726
because they're telling us to do it

00:02:56.726 --> 00:02:59.095
with a neat diagram.

00:02:59.095 --> 00:03:02.399
All right, let me draw it a
little bit better than that.

00:03:03.773 --> 00:03:05.480
It would look something like this.

00:03:05.480 --> 00:03:08.315
And then let me put a circle around this.

00:03:08.315 --> 00:03:11.277
So putting a circle around this,

00:03:11.277 --> 00:03:13.075
this is our incandescent lightbulb.

00:03:13.075 --> 00:03:14.480
This is our power source.

00:03:14.480 --> 00:03:16.443
To show it's a variable power source,

00:03:16.443 --> 00:03:19.194
I can put an arrow across it like this.

00:03:19.194 --> 00:03:21.516
That shows us that it is
a variable power source.

00:03:21.516 --> 00:03:25.023
So this is a circuit
that I've just set up.

00:03:25.023 --> 00:03:27.622
But I wanna use some
ammeters and voltmeters

00:03:27.622 --> 00:03:29.479
in order to measure what's happening

00:03:29.479 --> 00:03:33.962
as our electrons are going
through the lightbulb.

00:03:33.962 --> 00:03:35.633
The standard convention is to show current

00:03:35.633 --> 00:03:38.316
going from the positive terminal
to the negative terminal,

00:03:38.316 --> 00:03:39.674
but we know, and if you don't know

00:03:39.674 --> 00:03:40.939
I encourage you to watch the Khan Academy

00:03:40.939 --> 00:03:42.913
videos on it, what's actually happening

00:03:42.913 --> 00:03:44.318
is you have electrons traveling

00:03:44.318 --> 00:03:45.676
from the negative terminal

00:03:45.676 --> 00:03:47.441
to the positive one.

00:03:47.441 --> 00:03:48.637
But in general, if we just wanted to talk

00:03:48.637 --> 00:03:51.524
about the current, you would denote it,

00:03:52.464 --> 00:03:55.105
the convention is that the current

00:03:55.105 --> 00:03:57.160
goes from the positive direction

00:03:57.160 --> 00:03:57.996
to the negative direction.

00:03:57.996 --> 00:04:00.040
You could view it as
they're the positive gaps

00:04:00.040 --> 00:04:02.442
of electrons or however you want to,

00:04:02.442 --> 00:04:03.999
but the electrons are actually moving

00:04:03.999 --> 00:04:05.601
in the other direction.

00:04:05.601 --> 00:04:07.563
So the first question is do you have

00:04:07.563 --> 00:04:09.037
a different number of electrons

00:04:09.037 --> 00:04:11.417
moving per second before
entering the lightbulb

00:04:11.417 --> 00:04:13.879
then when you come out of the lightbulb?

00:04:13.879 --> 00:04:15.620
Well the way you can measure that

00:04:15.620 --> 00:04:16.723
is by measuring the current

00:04:16.723 --> 00:04:18.639
on either side of the lightbulb.

00:04:18.639 --> 00:04:19.939
The way we can measure the current

00:04:19.939 --> 00:04:21.077
on either side of the lightbulb

00:04:21.077 --> 00:04:26.077
is we can insert ammeters on either side.

00:04:26.406 --> 00:04:29.506
Ammeters have to be inserted in series.

00:04:29.506 --> 00:04:31.630
So I'm clearing up some space

00:04:31.630 --> 00:04:33.616
so I can insert my ammeters.

00:04:33.616 --> 00:04:35.651
So that is one ammeter.

00:04:35.651 --> 00:04:37.613
It's going to measure current

00:04:37.613 --> 00:04:38.845
right through that part.

00:04:38.845 --> 00:04:40.272
So I'll put A there.

00:04:40.272 --> 00:04:42.803
And then that is another ammeter

00:04:42.803 --> 00:04:45.566
right over there, ammeter.

00:04:45.566 --> 00:04:47.772
So these are going to measure current

00:04:47.772 --> 00:04:50.547
on either side of our lightbulb.

00:04:50.547 --> 00:04:55.547
So current measures current

00:04:57.738 --> 00:04:59.706
on either side.

00:05:02.210 --> 00:05:03.675
Or I could say measuring current

00:05:03.675 --> 00:05:06.677
entering and exiting lightbulb.

00:05:06.677 --> 00:05:11.677
Current entering and exiting

00:05:15.651 --> 00:05:18.333
the lightbulb.

00:05:18.333 --> 00:05:20.737
Entering the lightbulb,

00:05:20.737 --> 00:05:25.737
or maybe I'll say exiting the bulb.

00:05:25.857 --> 00:05:27.691
We also care about the voltage drop.

00:05:27.691 --> 00:05:29.293
So we could put a voltmeter,

00:05:29.293 --> 00:05:31.336
and the voltmeter can be put in parallel

00:05:31.336 --> 00:05:32.881
with the lightbulb.

00:05:32.881 --> 00:05:35.911
So let me draw the voltmeter here.

00:05:35.911 --> 00:05:39.245
So I'm trying to draw it neatly.

00:05:39.245 --> 00:05:43.169
So this is the voltmeter.

00:05:43.169 --> 00:05:45.061
It's going to measure the voltage drop

00:05:45.061 --> 00:05:47.005
from one side

00:05:48.240 --> 00:05:49.354
to another.

00:05:49.354 --> 00:05:50.971
Just connect this there.

00:05:50.971 --> 00:05:54.268
So this measures the voltage drop.

00:05:54.268 --> 00:05:59.268
Measures voltage.

00:06:00.550 --> 00:06:02.628
Measures voltage drop.

00:06:02.628 --> 00:06:03.812
So I've drawn my diagram.

00:06:03.812 --> 00:06:05.612
Let's see, what else do I need to do?

00:06:05.612 --> 00:06:07.806
So they say describe an
experimental procedure

00:06:07.806 --> 00:06:09.047
that could be used to answer questions

00:06:09.047 --> 00:06:10.883
one and two above.

00:06:10.883 --> 00:06:12.729
In your description,
state the measurements

00:06:12.729 --> 00:06:14.842
you would make and how you would use

00:06:14.842 --> 00:06:16.213
the equipment to make them.

00:06:16.213 --> 00:06:18.348
Include a neat, labeled
diagram of your setup.

00:06:18.348 --> 00:06:23.201
Okay, so I guess my description,

00:06:23.201 --> 00:06:28.201
I could say I'd put two ammeters

00:06:32.722 --> 00:06:35.397
in series

00:06:37.899 --> 00:06:40.994
with bulb.

00:06:42.879 --> 00:06:47.077
One before bulb.

00:06:52.494 --> 00:06:54.292
One after.

00:06:56.162 --> 00:06:57.981
If current same on both,

00:06:57.981 --> 00:06:59.630
then same number of electrons

00:06:59.630 --> 00:07:02.150
entering and exiting bulb.

00:07:02.150 --> 00:07:04.862
If the current

00:07:07.084 --> 00:07:09.075
is the same

00:07:11.565 --> 00:07:13.533
on either side,

00:07:18.102 --> 00:07:23.101
then electrons

00:07:24.696 --> 00:07:27.223
per second

00:07:28.134 --> 00:07:31.076
entering and exiting will be the same.

00:07:40.068 --> 00:07:42.583
If the currents are different,

00:07:51.667 --> 00:07:55.150
then the rate of electrons is different,

00:07:55.150 --> 00:08:00.150
then rate of electrons

00:08:01.616 --> 00:08:03.044
passing

00:08:05.185 --> 00:08:07.943
are different.

00:08:11.570 --> 00:08:13.231
All right, so that's the first part,

00:08:13.231 --> 00:08:15.668
the first statement, to try to go

00:08:15.668 --> 00:08:17.769
for this first statement
for statement one.

00:08:17.769 --> 00:08:19.152
In one second, do fewer electrons

00:08:19.152 --> 00:08:21.370
leave the bulb than enter the bulb?

00:08:21.370 --> 00:08:23.274
If the ammeters are
measuring the same current,

00:08:23.274 --> 00:08:24.329
well then you have the same number

00:08:24.329 --> 00:08:25.432
of electrons per second entering

00:08:25.432 --> 00:08:26.571
and leaving the bulb.

00:08:26.571 --> 00:08:28.428
If the ammeter measures
different currents,

00:08:28.428 --> 00:08:30.691
well then you've got different
numbers of electrons.

00:08:30.691 --> 00:08:33.093
All right, now for statement two,

00:08:33.093 --> 00:08:33.547
so let me write this,

00:08:33.547 --> 00:08:36.016
this is statement one.

00:08:39.922 --> 00:08:42.047
That is my procedure right over there.

00:08:42.047 --> 00:08:44.031
Now for statement two,

00:08:44.031 --> 00:08:46.331
statement one test I guess I could say,

00:08:46.331 --> 00:08:49.076
and then statement two test,

00:08:53.838 --> 00:08:54.951
I think you guys get the idea

00:08:54.951 --> 00:08:55.579
by this point,

00:08:55.579 --> 00:08:56.520
but I'm just writing it out

00:08:56.520 --> 00:08:57.541
because you would have to

00:08:57.541 --> 00:08:59.619
if you were taking this AP test,

00:08:59.619 --> 00:09:04.619
I would say put voltmeter,

00:09:07.537 --> 00:09:09.336
I could lowercase voltmeter,

00:09:09.336 --> 00:09:11.793
put voltmeter

00:09:13.923 --> 00:09:16.553
in parallel

00:09:17.603 --> 00:09:20.123
with, let me write it out,

00:09:20.123 --> 00:09:22.590
with bulb.

00:09:23.954 --> 00:09:28.954
If measures voltage drop,

00:09:35.633 --> 00:09:38.623
or I could say if and only if,

00:09:38.623 --> 00:09:40.541
I'll say if measured voltage drop,

00:09:40.541 --> 00:09:44.628
then, how did they actually
phrase the statement?

00:09:44.628 --> 00:09:46.333
Let's see, does the
electric potential energy

00:09:46.333 --> 00:09:48.610
of electrons change?

00:09:48.610 --> 00:09:51.199
Then electric potential energy

00:09:51.199 --> 00:09:53.446
of electron changes.

00:09:55.565 --> 00:09:57.775
Did they say electric potential

00:09:57.775 --> 00:09:59.923
energy of electron changes?

00:09:59.923 --> 00:10:04.923
Then electric potential

00:10:06.702 --> 00:10:08.485
energy

00:10:09.362 --> 00:10:12.770
of electrons,

00:10:13.774 --> 00:10:15.620
I don't normally write this much,

00:10:15.620 --> 00:10:17.675
changes.

00:10:17.675 --> 00:10:19.857
Otherwise, it does not.

00:10:19.857 --> 00:10:20.717
If no voltage drop,

00:10:20.717 --> 00:10:23.154
then no change in potential energy.

00:10:23.154 --> 00:10:26.330
If no voltage drop,

00:10:30.596 --> 00:10:35.136
then no change

00:10:35.136 --> 00:10:37.841
in potential energy,

00:10:37.841 --> 00:10:40.427
electric potential energy you could write.

00:10:42.638 --> 00:10:43.566
And there you go.

00:10:43.566 --> 00:10:45.296
That is part A

00:10:45.296 --> 00:10:47.340
where I've set up my neat diagram.

00:10:47.340 --> 00:10:49.117
I'm measuring the current entering

00:10:49.117 --> 00:10:50.555
and exiting the lightbulb.

00:10:50.555 --> 00:10:51.241
And actually, if you view it

00:10:51.241 --> 00:10:52.727
from the electrons point of view,

00:10:52.727 --> 00:10:53.865
this one is measuring the electrons

00:10:53.865 --> 00:10:55.583
going that direction,

00:10:55.583 --> 00:10:56.663
this is electrons entering,

00:10:56.663 --> 00:10:58.277
electrons exiting, but either way,

00:10:58.277 --> 00:10:59.205
the electrons per second

00:10:59.205 --> 00:11:00.067
would affect current.

00:11:00.067 --> 00:11:01.029
Same current?

00:11:01.029 --> 00:11:03.049
Well then you have the
same electrons per second.

00:11:03.049 --> 00:11:04.187
Different current, then you have different

00:11:04.187 --> 00:11:05.431
electrons per second entering

00:11:05.431 --> 00:11:06.650
and exiting the bulb.

00:11:06.650 --> 00:11:08.786
This measures the
electric potential energy

00:11:08.786 --> 00:11:10.330
across the lightbulb.

00:11:10.330 --> 00:11:14.788
If you actually measure a voltage here,

00:11:14.788 --> 00:11:17.308
then that means that the potential energy

00:11:17.308 --> 00:11:19.653
is changing from one side

00:11:19.653 --> 00:11:22.567
of the bulb to another.

00:11:22.567 --> 00:11:24.365
And so I'll stop there.

00:11:24.365 --> 00:11:27.849
Then I will,

00:11:27.849 --> 00:11:29.243
well, actually, I think
I answered part B too.

00:11:29.243 --> 00:11:31.122
Explain how data from the experiment

00:11:31.122 --> 00:11:32.087
you described can be used

00:11:32.087 --> 00:11:33.689
to answer question one above.

00:11:33.689 --> 00:11:34.943
Explain how the data from the experiment

00:11:34.943 --> 00:11:36.023
you described can be used

00:11:36.023 --> 00:11:37.404
to answer question two above.

00:11:37.404 --> 00:11:39.888
So this was really parts A and B.

00:11:39.888 --> 00:11:41.107
So actually let me write this down.

00:11:41.107 --> 00:11:44.893
So this is A plus B right over here.

00:11:44.893 --> 00:11:45.844
We have the diagram.

00:11:45.844 --> 00:11:46.588
If you're taking the test,

00:11:46.588 --> 00:11:48.248
depending on how your time pressure is,

00:11:48.248 --> 00:11:50.024
you might want to label this more,

00:11:50.024 --> 00:11:50.988
but if you're running out of time,

00:11:50.988 --> 00:11:52.090
then you might not have time

00:11:52.090 --> 00:11:54.801
to describe it in as much detail.


Avoiding common mistakes in historical essays

WEBVTT
Kind: captions
Language: en

00:00:00.229 --> 00:00:02.129
- [Voiceover] I want to talk about how to
avoid

00:00:02.129 --> 00:00:05.270
some common mistakes when you're writing

00:00:05.270 --> 00:00:06.960
a historical paper.

00:00:06.960 --> 00:00:11.960
This could apply to a term
paper, to a blue book essay,

00:00:12.329 --> 00:00:15.839
even really to your Master's thesis if you
wanted to.

00:00:15.839 --> 00:00:18.619
I want to talk about three phrases

00:00:18.619 --> 00:00:22.609
that you might be tempted
to use in a historical essay

00:00:22.609 --> 00:00:25.570
that actually muddy what you're trying to
say

00:00:25.570 --> 00:00:29.039
and undercut your point
more than it helps.

00:00:29.039 --> 00:00:33.490
So, these three phrases that
I want to talk about are:

00:00:35.280 --> 00:00:36.950
"Throughout history,"

00:00:45.790 --> 00:00:47.349
"It was inevitable,"

00:00:53.889 --> 00:00:57.390
and "And that's why

00:01:00.860 --> 00:01:02.270
[insert country here]

00:01:10.030 --> 00:01:11.430
is so great today."

00:01:18.540 --> 00:01:21.880
So, why are these phrases so problematic?

00:01:21.880 --> 00:01:24.290
Let's start with "Throughout history."

00:01:24.290 --> 00:01:29.090
So, this is something that you frequently
see in writing,

00:01:29.090 --> 00:01:34.040
from historical essays
to pieces of journalism

00:01:34.040 --> 00:01:37.970
and it often has the ring
of making something seem

00:01:37.970 --> 00:01:42.970
really strong and adding the
weight of eons of history

00:01:43.550 --> 00:01:46.170
behind a single sentence.

00:01:46.170 --> 00:01:48.150
History is a very long thing.

00:01:48.150 --> 00:01:50.680
I mean, for recorded
history we're going back

00:01:50.680 --> 00:01:54.360
maybe five thousand, seven thousand years.

00:01:54.360 --> 00:01:56.420
And think about the
many different cultures

00:01:56.420 --> 00:01:59.670
and types of people and ideas that existed

00:01:59.670 --> 00:02:01.340
throughout that time period.

00:02:01.340 --> 00:02:04.540
If you're sitting down to write
an essay about, let's say,

00:02:04.540 --> 00:02:08.649
The Cold War, and you
start: "Throughout history

00:02:08.649 --> 00:02:11.388
people have feared nuclear attack,"

00:02:11.388 --> 00:02:13.950
well, the first thing your reader is going
to think is:

00:02:13.950 --> 00:02:17.779
"Wait! The nuclear bomb
was only developed in 1945.

00:02:17.779 --> 00:02:19.890
I mean, that's not throughout history,

00:02:19.890 --> 00:02:22.280
that's only throughout
the last seventy years."

00:02:22.280 --> 00:02:25.609
Or what about: "Throughout
history people have gone

00:02:25.609 --> 00:02:27.750
to war over religion."

00:02:27.750 --> 00:02:31.359
Your reader might think:
"Well, what about when people

00:02:31.359 --> 00:02:34.239
didn't live close enough to each other to
go to war

00:02:34.239 --> 00:02:36.040
about different beliefs?"

00:02:36.040 --> 00:02:37.879
And do we really want to send the message

00:02:37.879 --> 00:02:41.180
that having different religions mean that
you necessarily

00:02:41.180 --> 00:02:42.599
have to go to war?

00:02:42.599 --> 00:02:44.510
One thing that "Throughout
history" does is

00:02:44.510 --> 00:02:48.569
it makes an assumption
about human nature, right?,

00:02:48.569 --> 00:02:53.569
that the way that people think now is the
way that people

00:02:53.620 --> 00:02:56.269
have always thought throughout history,

00:02:56.269 --> 00:02:59.670
or the way that people behave
now is the way that people

00:02:59.670 --> 00:03:02.510
have always behaved throughout history.

00:03:02.510 --> 00:03:05.749
And if there's anything that is one of the
core beliefs

00:03:05.749 --> 00:03:09.040
of the study of history is that people are
different

00:03:09.040 --> 00:03:10.239
over time.

00:03:10.239 --> 00:03:13.340
It's fun to study the past because people
in the past

00:03:13.340 --> 00:03:14.829
weren't like us.

00:03:14.829 --> 00:03:18.319
They had different
ideas, different beliefs,

00:03:18.319 --> 00:03:20.349
different cultural values.

00:03:20.349 --> 00:03:22.380
And so, if you want to be really strong

00:03:22.380 --> 00:03:24.900
about how you start a historical essay,

00:03:24.900 --> 00:03:28.019
always start it in a really specific part

00:03:28.019 --> 00:03:30.180
of time that you're talking about.

00:03:30.180 --> 00:03:35.180
So, if you're talking about
the period from 1945 to 1965,

00:03:36.849 --> 00:03:38.709
say "In the post-war era"

00:03:47.979 --> 00:03:51.509
or "In the late 19th century."

00:03:57.739 --> 00:04:00.839
And you might also add
"In the United States."

00:04:02.049 --> 00:04:02.829
Right?

00:04:02.829 --> 00:04:07.680
This shows that you have a
strong grasp of both the time

00:04:07.680 --> 00:04:10.010
and the place that you're writing about.

00:04:10.010 --> 00:04:12.819
And so, you can make an
argument that is specific

00:04:12.819 --> 00:04:14.389
to that time period.

00:04:14.389 --> 00:04:17.220
Okay, let's move on to
"It was inevitable."

00:04:17.220 --> 00:04:21.070
I think we like to use the word inevitable
because it's long

00:04:21.070 --> 00:04:23.190
and it sounds pretty cool.

00:04:23.190 --> 00:04:26.160
But think about what
inevitable really means.

00:04:26.160 --> 00:04:30.980
It means "It was unavoidable,"
there was no other thing

00:04:30.980 --> 00:04:32.570
that could have happened.

00:04:32.570 --> 00:04:35.920
Now think of a version of
history where everything

00:04:35.920 --> 00:04:37.250
is inevitable.

00:04:37.250 --> 00:04:40.530
Everything was just
going to happen no matter

00:04:40.530 --> 00:04:43.340
whether anyone did anything or not.

00:04:43.340 --> 00:04:45.820
That shows an interpretation
of history that says

00:04:45.820 --> 00:04:48.420
that people's choices don't matter.

00:04:49.330 --> 00:04:52.440
And if you want to emphasize
anything in history,

00:04:52.440 --> 00:04:54.900
it's how much choices matter.

00:04:58.370 --> 00:05:02.920
There are very few things that
are inevitable in history.

00:05:02.920 --> 00:05:07.420
Most of them, I would say,
are natural disasters, right?

00:05:07.420 --> 00:05:11.200
There is going to eventually
be an earthquake in California

00:05:11.200 --> 00:05:14.180
is inevitable because
there's a fault line.

00:05:14.180 --> 00:05:16.540
That's something that
humans can't control,

00:05:16.540 --> 00:05:19.140
But for almost everything else in history,

00:05:19.140 --> 00:05:21.130
humans can control it.

00:05:21.130 --> 00:05:25.310
And they do decide how to
react to certain situations.

00:05:25.310 --> 00:05:30.130
For example, take the
sinking of the USS Maine

00:05:30.130 --> 00:05:32.300
in Havana Harbor, right?

00:05:32.300 --> 00:05:35.840
This is the event that leads to the United
States going

00:05:35.840 --> 00:05:40.150
to war with Spain over Cuba in 1898.

00:05:40.150 --> 00:05:41.920
And the reason that this happened was

00:05:41.920 --> 00:05:46.230
because the USS Maine
exploded in Havana Harbor.

00:05:46.230 --> 00:05:50.120
Now, we know and the Spanish
suggested at the time

00:05:50.120 --> 00:05:53.700
that the reason that the
USS Maine exploded was due

00:05:53.700 --> 00:05:56.560
to a spontaneous combustion on-board.

00:05:56.560 --> 00:05:58.490
There was an equipment malfunction.

00:05:58.490 --> 00:06:02.550
The United States chose to
believe that this ship sinking

00:06:02.550 --> 00:06:06.110
was the result of a Spanish
bomb and declared war.

00:06:06.110 --> 00:06:09.240
Now, you might have said:
"War was inevitable."

00:06:09.240 --> 00:06:10.590
But it really wasn't.

00:06:10.590 --> 00:06:12.210
There were many ways
that the United States

00:06:12.210 --> 00:06:16.010
could have chosen differently
in that moment, to say:

00:06:16.010 --> 00:06:19.230
"Maybe we will believe the
Spanish and just leave it alone"

00:06:19.230 --> 00:06:22.860
or "Maybe we'll send some
financial aid to Cuba

00:06:22.860 --> 00:06:25.020
but we don't have to go to war."

00:06:25.020 --> 00:06:29.140
When you get rid of inevitability
in history, you open up

00:06:29.140 --> 00:06:33.800
new choices, new ways that
things could have gone.

00:06:33.800 --> 00:06:38.010
And that is really the heart of
history, it's the possibility for

00:06:38.010 --> 00:06:40.240
things to be different than they were

00:06:40.240 --> 00:06:42.360
and different than they are.

00:06:42.360 --> 00:06:45.500
Okay, let's finish up with "And that's why

00:06:45.500 --> 00:06:48.700
[insert country here] is so great today."

00:06:48.700 --> 00:06:51.680
You see this all the time
in historical papers.

00:06:51.680 --> 00:06:54.960
And I think writers are
very tempted to finish

00:06:54.960 --> 00:06:59.660
a historical essay with some
expression of patriotism.

00:06:59.660 --> 00:07:02.910
And maybe in a few rare
cases this is true.

00:07:02.910 --> 00:07:06.620
You could say: "The United
States is a better place today

00:07:06.620 --> 00:07:11.620
than it was in the 1950s
thanks to the Civil Rights Act

00:07:12.710 --> 00:07:14.420
of 1964."

00:07:14.420 --> 00:07:17.110
But it's something you
want to use very sparingly,

00:07:17.110 --> 00:07:20.640
because usually the scope of a historical
paper - and think

00:07:20.640 --> 00:07:25.640
about our "Throughout
history" here - isn't so huge

00:07:25.800 --> 00:07:29.270
as to merit the reaction to it being

00:07:29.270 --> 00:07:33.500
"This is the heart of what
makes America great today."

00:07:33.500 --> 00:07:37.400
I've read historical papers
about the Spanish Flu in 1919

00:07:37.400 --> 00:07:40.390
that end with "And that's
why America is great today."

00:07:40.390 --> 00:07:43.070
Ask yourself: is this relevant?

00:07:43.070 --> 00:07:46.560
And even if it is, is it the most relevant
way

00:07:46.560 --> 00:07:48.450
that you could end an essay?

00:07:48.450 --> 00:07:51.970
For a historical essay you
want to keep your conclusions

00:07:51.970 --> 00:07:54.240
very specific, the same
way that you want to keep

00:07:54.240 --> 00:07:56.360
your period of time specific.

00:07:56.360 --> 00:08:00.080
So, if you're talking about
the post-war era, conclude

00:08:00.080 --> 00:08:03.220
with something that you
can actually substantiate,

00:08:03.220 --> 00:08:06.100
that you have substantiated in your essay,

00:08:06.100 --> 00:08:11.090
about the post-war era,
say, "The wealth generated

00:08:11.090 --> 00:08:15.930
by industrialization after
World War II was the reason

00:08:15.930 --> 00:08:18.110
that the Baby Boom happened."

00:08:18.110 --> 00:08:21.010
Don't say "And that's why
America is great today."

00:08:21.010 --> 00:08:23.980
What do you mean by great?

00:08:23.980 --> 00:08:27.360
Do you mean economically great?

00:08:27.360 --> 00:08:28.730
Culturally Great?

00:08:28.730 --> 00:08:30.440
Politically great?

00:08:30.440 --> 00:08:34.289
It's a little too vague, and vagueness can
really undermine

00:08:34.289 --> 00:08:36.989
your argument as opposed to supporting it.

00:08:36.989 --> 00:08:41.989
In a way these are all kind of appeals for
human nature,

00:08:42.779 --> 00:08:47.779
appeals for the natural
progress of history,

00:08:48.170 --> 00:08:52.850
and appeals to patriotism
that are less rooted

00:08:52.850 --> 00:08:56.990
in the fact of what you want
to say than they are rooted

00:08:56.990 --> 00:09:01.019
in ways of trying to get
your reader sympathy.

00:09:01.019 --> 00:09:06.019
Instead what you can do is be specific in
your time

00:09:06.139 --> 00:09:08.020
and your place.

00:09:09.190 --> 00:09:12.249
Emphasize choices and points where things

00:09:12.249 --> 00:09:14.630
might have gone differently than they did.

00:09:17.680 --> 00:09:21.350
And end with a conclusion
that is very related

00:09:21.350 --> 00:09:24.859
to the things that you
specifically addressed.

00:09:24.859 --> 00:09:27.759
Remember, you never want to
introduce new information

00:09:27.759 --> 00:09:31.019
in your conclusion, and
saying "And that's why America

00:09:31.019 --> 00:09:33.860
is great today" is new information,

00:09:33.860 --> 00:09:36.699
because it might not
necessarily be related.

00:09:36.699 --> 00:09:40.699
Instead think about what it
was you proved in this paper

00:09:40.699 --> 00:09:43.240
and key your conclusion directly to that.


How to read a document

WEBVTT
Kind: captions
Language: en

00:00:00.109 --> 00:00:00.910
- [Voiceover] Hello David.

00:00:00.910 --> 00:00:01.967
- [Voiceover] Hello Kim.

00:00:01.967 --> 00:00:03.766
- [Voiceover] So today what we're doing

00:00:03.766 --> 00:00:06.247
is taking a look at this speech

00:00:06.247 --> 00:00:08.526
by one of my favorite Presidents,

00:00:08.526 --> 00:00:11.347
Franklin Delano Roosevelt which he gave

00:00:11.347 --> 00:00:14.424
at his inauguration in 1933.

00:00:14.424 --> 00:00:16.282
And I think what's really important

00:00:16.282 --> 00:00:18.128
about looking at a speech like this

00:00:18.128 --> 00:00:20.821
is not only that we can
learn to analyze this

00:00:20.821 --> 00:00:23.422
as a primary source, which will be helpful

00:00:23.422 --> 00:00:26.011
for thinking about it historically,

00:00:26.011 --> 00:00:28.124
but also because I
think it's really useful

00:00:28.124 --> 00:00:30.527
to be able to look at
a Presidential speech,

00:00:30.527 --> 00:00:32.629
or a speech given by any politician,

00:00:32.629 --> 00:00:35.125
and understand what kind
of claims they're making

00:00:35.125 --> 00:00:37.365
and how they're making them.

00:00:37.365 --> 00:00:39.490
- [Voiceover] So, Kim,
before we go any further,

00:00:39.490 --> 00:00:41.870
what even is a primary source?

00:00:41.870 --> 00:00:43.031
What's the difference between a primary

00:00:43.031 --> 00:00:44.947
and a secondary source?

00:00:44.947 --> 00:00:45.667
- [Voiceover] Great question.

00:00:45.667 --> 00:00:49.816
So a primary source is a document

00:00:49.816 --> 00:00:53.724
that takes a look at an event

00:00:53.724 --> 00:00:57.060
from the perspective of
someone who was there.

00:00:57.060 --> 00:01:01.277
So, a primary source
could be lots of things.

00:01:01.277 --> 00:01:05.840
It could be a photograph

00:01:05.840 --> 00:01:08.969
taken by someone who was, perhaps,

00:01:08.969 --> 00:01:12.369
attending a political rally.

00:01:12.369 --> 00:01:15.535
It could be a diary of, maybe,

00:01:15.535 --> 00:01:19.847
someone who was active in
the women's rights movement

00:01:19.847 --> 00:01:22.747
in the 19th century.

00:01:22.747 --> 00:01:27.149
Certainly any speech,

00:01:27.149 --> 00:01:31.781
or even, let's say, like a
oral history conversation,

00:01:31.781 --> 00:01:34.382
and I've mentioned a lot
of significant things here,

00:01:34.382 --> 00:01:37.283
but it also doesn't even
have to be something that is

00:01:37.283 --> 00:01:41.464
connected with a significant
person or a famous event.

00:01:41.464 --> 00:01:46.206
It could be a shopping list, right,

00:01:46.206 --> 00:01:49.545
if you are studying the consumption habits

00:01:49.545 --> 00:01:51.507
of someone who lived in the 1950's.

00:01:51.507 --> 00:01:54.377
What they bought at the grocery
store would tell you a lot

00:01:54.377 --> 00:01:58.441
about what they ate,
what they could spend.

00:01:58.441 --> 00:02:02.362
So, a primary source is
kind of the real meat

00:02:02.362 --> 00:02:06.240
of research material that
shows you what people,

00:02:06.240 --> 00:02:08.446
at the time, were thinking.

00:02:08.446 --> 00:02:10.826
- [Voiceover] Okay, so a
primary source is an artifact

00:02:10.826 --> 00:02:13.786
left behind by someone who was there.

00:02:13.786 --> 00:02:14.402
- [Voiceover] Exactly.

00:02:14.402 --> 00:02:15.830
- [Voiceover] What is a secondary source?

00:02:15.830 --> 00:02:19.649
- [Voiceover] So a secondary
source is an interpretation.

00:02:19.649 --> 00:02:22.691
So, say I'm a historian,

00:02:22.691 --> 00:02:25.071
which I happen to be.

00:02:25.071 --> 00:02:25.907
- [Voiceover] Oh my goodness!

00:02:25.907 --> 00:02:26.906
(laughter)

00:02:26.906 --> 00:02:27.823
What a coincidence.

00:02:27.823 --> 00:02:29.376
(laughter)

00:02:29.376 --> 00:02:31.503
- [Voiceover] So I have
done the work of digging up

00:02:31.503 --> 00:02:34.603
a bunch of primary sources, and, then,

00:02:34.603 --> 00:02:38.121
you look at all of them and
see what they have in common,

00:02:38.121 --> 00:02:39.305
for example.

00:02:39.305 --> 00:02:42.405
So maybe I'm writing
about Abraham Lincoln,

00:02:42.405 --> 00:02:45.389
and I get a lot of photographs of Lincoln,

00:02:45.389 --> 00:02:49.824
I get a lot of writings by
Lincoln and his contemporaries

00:02:49.824 --> 00:02:53.423
and I go through all of them and I come up

00:02:53.423 --> 00:02:55.907
with my interpretation
of what was going on

00:02:55.907 --> 00:02:57.300
in Lincoln's life.

00:02:57.300 --> 00:03:00.670
So, I write a book on Lincoln by Kim.

00:03:02.050 --> 00:03:03.341
- [Voiceover] Until now...

00:03:03.341 --> 00:03:05.160
(laughter)

00:03:05.160 --> 00:03:08.968
- [Voiceover] And that's
my interpretation.

00:03:08.968 --> 00:03:09.468
- [Voiceover] Okay.

00:03:09.468 --> 00:03:09.908
- [Voiceover] Right?

00:03:09.908 --> 00:03:12.324
So the things that I'm interested in

00:03:12.324 --> 00:03:15.911
say Lincoln's religion, or lack thereof,

00:03:15.911 --> 00:03:17.809
might not be the same things

00:03:17.809 --> 00:03:20.010
that another historian
would be interested.

00:03:20.010 --> 00:03:23.002
Say, they're interested in
Lincoln's foreign policy.

00:03:23.002 --> 00:03:25.768
So, my interpretation is just one way

00:03:25.768 --> 00:03:27.881
of looking at those primary sources

00:03:27.881 --> 00:03:29.228
where another historian might have

00:03:29.228 --> 00:03:32.188
a completely different interpretation.

00:03:32.188 --> 00:03:34.553
What's also important
about secondary sources is

00:03:34.553 --> 00:03:36.949
that I wasn't there, right?

00:03:36.949 --> 00:03:38.028
I never talked to Lincoln.

00:03:38.028 --> 00:03:41.523
He, you know, died more than
100 years before I was born,

00:03:41.523 --> 00:03:45.366
which means that you can
only trust me so much.

00:03:45.366 --> 00:03:48.303
You can, instead, maybe
get a much clearer picture

00:03:48.303 --> 00:03:50.068
of what Lincoln was really thinking

00:03:50.068 --> 00:03:52.590
by reading his own words.

00:03:52.590 --> 00:03:53.783
- [Voiceover] So, trust secondary sources

00:03:53.783 --> 00:03:55.884
about as far as you can throw them?

00:03:55.884 --> 00:03:57.370
- [Voiceover] Well,
maybe trust all sources

00:03:57.370 --> 00:03:59.809
about as far as you can throw them, right,

00:03:59.809 --> 00:04:03.200
because everyone at every time

00:04:03.200 --> 00:04:05.370
has their own perspective.

00:04:05.370 --> 00:04:09.909
And so, the ideas of someone
who lived in the 19th century

00:04:09.909 --> 00:04:11.790
are gonna be different than the ideas

00:04:11.790 --> 00:04:13.880
of someone who lives now,

00:04:13.880 --> 00:04:16.411
and you only know as much
as you can know, right?

00:04:16.411 --> 00:04:20.544
You're only as informed as
the information that you have.

00:04:20.544 --> 00:04:24.027
So, you really have to take
everything with a grain of salt

00:04:24.027 --> 00:04:26.720
and compare it with other
sources from its time period,

00:04:26.720 --> 00:04:28.903
and other sources later on, to get a sense

00:04:28.903 --> 00:04:31.283
of what's important.

00:04:31.283 --> 00:04:32.851
- [Voiceover] So you're
saying that you might have

00:04:32.851 --> 00:04:34.766
a different perspective on Lincoln

00:04:34.766 --> 00:04:36.647
than another Lincoln scholar, but

00:04:36.647 --> 00:04:39.480
that Lincoln's writings,
themselves, also contain

00:04:39.480 --> 00:04:42.487
Lincoln's own biases from his lifetime.

00:04:42.487 --> 00:04:43.009
- [Voiceover] Right.

00:04:43.009 --> 00:04:44.407
- [Voiceover] Okay, so what are we doing

00:04:44.407 --> 00:04:47.003
with Roosevelt's Inaugural Address, here?

00:04:47.003 --> 00:04:47.421
- [Voiceover] All right.

00:04:47.421 --> 00:04:51.112
So, let's take a look at
this Inaugural Address

00:04:51.112 --> 00:04:53.447
as though we're historians, right?

00:04:53.447 --> 00:04:58.326
We're gonna sit down and
really get into the...

00:04:58.326 --> 00:05:00.180
- [Voiceover] The feeling
of the Great Depression?

00:05:00.180 --> 00:05:02.131
(laughter)

00:05:02.131 --> 00:05:02.886
All right.

00:05:02.886 --> 00:05:04.047
- [Voiceover] We're gonna get depressed.

00:05:04.047 --> 00:05:04.964
- [Voiceover] All right, I'm ready.

00:05:04.964 --> 00:05:07.820
So we've determined that
because he was there

00:05:07.820 --> 00:05:09.840
and because this is a
speech delivered by him,

00:05:09.840 --> 00:05:12.185
that this speech of
Franklin Delano Roosevelt

00:05:12.185 --> 00:05:13.509
is a primary source.

00:05:13.509 --> 00:05:15.088
- [Voiceover] Right, and
it's a great way to look

00:05:15.088 --> 00:05:16.446
at the Great Depression, right?

00:05:16.446 --> 00:05:18.942
If we want to know what
people are thinking about,

00:05:18.942 --> 00:05:21.264
it's very important to
see what the President

00:05:21.264 --> 00:05:24.910
of the United States has to
say when he's been elected.

00:05:24.910 --> 00:05:27.185
So David, I know that you've
been dying to read this

00:05:27.185 --> 00:05:29.959
in your terrific impression of Roosevelt,

00:05:29.959 --> 00:05:32.305
so I'm gonna turn it over
to you to get a sense

00:05:32.305 --> 00:05:34.151
of what Roosevelt has to say.

00:05:34.151 --> 00:05:35.904
- [Voiceover] Okay, I'm gonna
scoot back from the mike.

00:05:35.904 --> 00:05:38.342
"I am certain that my
fellow Americans expect

00:05:38.342 --> 00:05:40.740
"that on my induction into the Presidency

00:05:40.740 --> 00:05:43.149
"I will address them with
a candor and a decision

00:05:43.149 --> 00:05:46.040
"which the present situation
of our people impel.

00:05:46.040 --> 00:05:48.162
"This is preeminently the
time to speak the truth,

00:05:48.162 --> 00:05:50.707
"the whole truth, frankly and boldly.

00:05:50.707 --> 00:05:53.087
"Nor need we shrink from
honestly facing conditions

00:05:53.087 --> 00:05:54.689
"in our country today.

00:05:54.689 --> 00:05:57.428
"This great Nation will
endure as it has endured,

00:05:57.428 --> 00:05:59.705
"will revive and will prosper.

00:05:59.705 --> 00:06:02.271
"So, first of all, let
me assert my firm belief

00:06:02.271 --> 00:06:06.102
"that the only thing we have
to fear is fear itself -

00:06:06.102 --> 00:06:09.805
"nameless, unreasoning, unjustified terror

00:06:09.805 --> 00:06:11.442
"which paralyzes needed efforts

00:06:11.442 --> 00:06:13.962
"to convert retreat into advance."

00:06:13.962 --> 00:06:15.785
- [Voiceover] That was
beautiful (laughter).

00:06:15.785 --> 00:06:17.584
Thank you so much.

00:06:17.584 --> 00:06:18.258
- [Voiceover] You're welcome.

00:06:18.258 --> 00:06:18.850
- [Voiceover] All right.

00:06:18.850 --> 00:06:22.031
So how do we analyze
this as a primary source

00:06:22.031 --> 00:06:23.491
and as a speech.

00:06:23.491 --> 00:06:25.356
And I think the first thing we want to do,

00:06:25.356 --> 00:06:30.085
step one if you will, is just
identify what's going on, and

00:06:30.085 --> 00:06:32.834
thankfully, that's pretty
easy for us, right now.

00:06:32.834 --> 00:06:35.774
- [Voiceover] Right, this is a
speech given by the President

00:06:35.774 --> 00:06:37.504
of the United States in the moment

00:06:37.504 --> 00:06:39.164
that he becomes President.

00:06:39.164 --> 00:06:42.113
- [Voiceover] Right,
so we know when it was,

00:06:42.113 --> 00:06:45.260
in March 4th, 1933.

00:06:45.260 --> 00:06:49.039
We know who gave this speech,

00:06:49.039 --> 00:06:50.150
Franklin Delano Roosevelt,

00:06:50.150 --> 00:06:52.592
just about to be inducted as President.

00:06:52.592 --> 00:06:55.514
We know why he gave it,
right, very important

00:06:55.514 --> 00:06:58.509
for Presidents when they take office

00:06:58.509 --> 00:07:00.831
to make an Inaugural Address.

00:07:00.831 --> 00:07:02.735
So, we've got some basics here.

00:07:02.735 --> 00:07:05.893
We can even infer from
the Inaugural Address

00:07:05.893 --> 00:07:09.578
where this was given,
right, in Washington D.C.

00:07:13.668 --> 00:07:15.739
All right so in our identification,

00:07:15.739 --> 00:07:18.746
we've got that it's a
speech, it's in D.C.,

00:07:18.746 --> 00:07:22.438
happened in 1933, by FDR.

00:07:22.438 --> 00:07:25.416
So that's our identification stage.

00:07:28.846 --> 00:07:31.087
So to get at a little
deeper level for this,

00:07:31.087 --> 00:07:33.862
let's move on to a second step

00:07:33.862 --> 00:07:36.885
which would be, kind
of giving some context.

00:07:38.065 --> 00:07:41.533
So it's 1933. What's going on?

00:07:41.533 --> 00:07:42.314
- [Voiceover] Let's see.

00:07:42.314 --> 00:07:46.609
So, the Great Depression has
been going on for four years.

00:07:46.609 --> 00:07:47.654
- [Voiceover] Uh-hmm.

00:07:47.654 --> 00:07:49.872
- [Voiceover] Prohibition
has not ended yet, right?

00:07:49.872 --> 00:07:50.514
- [Voiceover] Right.

00:07:50.514 --> 00:07:51.753
- [Voiceover] Repeal has not come,

00:07:51.753 --> 00:07:54.671
so liquor is still illegal
in the United States,

00:07:54.671 --> 00:07:58.092
for sale and transport.

00:07:58.092 --> 00:07:59.717
There's massive unemployment.

00:07:59.717 --> 00:08:01.331
The Dust Bowl is still raging.

00:08:01.331 --> 00:08:03.990
America is not in the greatest place!

00:08:03.990 --> 00:08:05.778
- [Voiceover] No, it's a depression,

00:08:05.778 --> 00:08:09.214
and it's a depression in
all sorts of ways, right.

00:08:09.214 --> 00:08:11.130
People are emotionally depressed

00:08:11.130 --> 00:08:13.312
and there's an economic depression.

00:08:13.312 --> 00:08:15.158
All right, so we've got
the general gist now

00:08:15.158 --> 00:08:18.406
that this is a speech from 1933

00:08:18.406 --> 00:08:20.194
confronting the Great Depression.

00:08:20.194 --> 00:08:22.632
So let's get into a little
bit more of the specifics.

00:08:22.632 --> 00:08:26.403
What is he actually talking
about in this speech?

00:08:27.253 --> 00:08:28.553
- [Voiceover] Well, if
you look at this speech,

00:08:28.553 --> 00:08:32.593
you can kind of see
that he's acknowledging

00:08:32.593 --> 00:08:34.114
that things are bad.

00:08:34.114 --> 00:08:34.666
- [Voiceover] Right.

00:08:34.666 --> 00:08:37.446
- [Voiceover] Right. It's
time to speak the truth.

00:08:37.446 --> 00:08:39.292
So he keeps talking about how, you know,

00:08:39.292 --> 00:08:40.268
"It's time to speak the truth.

00:08:40.268 --> 00:08:42.532
"We'll address the American
people with candor.

00:08:42.532 --> 00:08:44.215
"It is time to speak the
truth, the whole truth,

00:08:44.215 --> 00:08:45.852
"frankly and boldly.

00:08:45.852 --> 00:08:48.789
"We will not shrink from
honestly facing conditions

00:08:48.789 --> 00:08:50.552
"in the country today."

00:08:50.552 --> 00:08:53.445
So Roosevelt is really
priming everyone to say,

00:08:53.445 --> 00:08:56.452
"Like okay, you have
not been told the truth

00:08:56.452 --> 00:08:59.053
"from your head of government
for the longest time,

00:08:59.053 --> 00:09:01.212
"and now it's time to deal frankly

00:09:01.212 --> 00:09:03.673
"with just how bad things have gotten."

00:09:03.673 --> 00:09:05.784
And what's interesting, is that he says,

00:09:05.784 --> 00:09:09.513
"Things are not, you know, great,

00:09:09.513 --> 00:09:13.379
"but in every dark hour
of our National life

00:09:13.379 --> 00:09:15.295
"a leadership of frankness
and vigor has met

00:09:15.295 --> 00:09:16.688
"with that understanding and support

00:09:16.688 --> 00:09:19.254
"of the people themselves
which is essential to victory."

00:09:19.254 --> 00:09:22.249
And he's saying that
there's no need to be afraid

00:09:22.249 --> 00:09:25.616
of anything except just malaise.

00:09:25.616 --> 00:09:28.194
He's saying that Americans
need to meet the problem

00:09:28.194 --> 00:09:32.858
of the depression with like an
up-welling of national will.

00:09:32.858 --> 00:09:35.218
- [Voiceover] Right, and I
think, you know, it's nice

00:09:35.218 --> 00:09:38.225
that he's saying, "Look, I'm
gonna tell it like it is.

00:09:38.225 --> 00:09:39.142
"Things are bad.

00:09:39.142 --> 00:09:40.999
"I recognize that things are bad."

00:09:40.999 --> 00:09:42.323
And that's pretty important,

00:09:42.323 --> 00:09:45.957
because up until this point,
Herbert Hoover hadn't really

00:09:45.957 --> 00:09:48.024
done much to recognize
that things were bad.

00:09:48.024 --> 00:09:51.158
You know, he saw that people
were suffering, and, yet,

00:09:51.158 --> 00:09:53.155
he said this is not
necessarily the responsibility

00:09:53.155 --> 00:09:55.477
of government to deal with this crisis.

00:09:55.477 --> 00:09:57.462
- [Voiceover] So Roosevelt,
actually, calls it,

00:09:57.462 --> 00:09:59.680
"a dark hour of our National life."

00:09:59.680 --> 00:10:01.479
Right, like this is an acknowledging

00:10:01.479 --> 00:10:04.161
that things are not great is
a big part of this speech.

00:10:04.161 --> 00:10:05.914
But he's also saying
that it's possible for us

00:10:05.914 --> 00:10:08.963
to bounce back if we are
honest about the problems,

00:10:08.963 --> 00:10:10.837
and we address it with vigor,

00:10:10.837 --> 00:10:13.356
and that is kind of the New Deal, right,

00:10:13.356 --> 00:10:14.959
is addressing the problems honestly

00:10:14.959 --> 00:10:17.304
and with national exuberance.

00:10:17.304 --> 00:10:18.225
- [Voiceover] Yeah, and I think

00:10:18.225 --> 00:10:21.210
this is such a fascinating speech

00:10:21.210 --> 00:10:25.199
because, for one thing,
this phrase has kind of come

00:10:25.199 --> 00:10:27.544
into our national lexicon, right.

00:10:27.544 --> 00:10:30.353
"There's nothing to fear but fear itself,"

00:10:30.353 --> 00:10:31.874
which is kind of strange.

00:10:31.874 --> 00:10:33.059
It's one of those things,

00:10:33.059 --> 00:10:34.742
like "Have your cake and eat it too."

00:10:34.742 --> 00:10:37.343
That you're like, "Wait
how is that possible?"

00:10:37.343 --> 00:10:39.569
So, what does he mean by,

00:10:39.569 --> 00:10:43.148
"The only thing we have
to fear is fear itself."

00:10:43.148 --> 00:10:43.763
- [Voiceover] I think he's saying

00:10:43.763 --> 00:10:45.806
that this is no time to panic,

00:10:45.806 --> 00:10:47.618
and that the only thing
that we should be afraid of

00:10:47.618 --> 00:10:50.299
is unreasoning terror.

00:10:50.299 --> 00:10:51.507
We shouldn't be running
around like chickens

00:10:51.507 --> 00:10:53.318
with our heads cut off.

00:10:53.318 --> 00:10:55.512
Right, like this is the time to stand firm

00:10:55.512 --> 00:10:58.972
against nameless terror and
focus on making the problems

00:10:58.972 --> 00:11:01.145
that we are facing into small,

00:11:01.145 --> 00:11:03.581
like accessible, combatable chunks.

00:11:03.581 --> 00:11:05.239
- [Voiceover] I think that
another thing that's important

00:11:05.239 --> 00:11:06.702
about what he's saying there is

00:11:06.702 --> 00:11:10.952
that the Great Depression
is caused by something

00:11:10.952 --> 00:11:14.141
that is very new in American culture,

00:11:14.141 --> 00:11:15.981
which is the stock market.

00:11:15.981 --> 00:11:18.592
And the stock market
doesn't play by the rules

00:11:18.592 --> 00:11:21.124
of straight supply and demand.

00:11:21.124 --> 00:11:23.237
Instead, they play on confidence.

00:11:23.237 --> 00:11:26.581
And so, the reason that
the stock market crash

00:11:26.581 --> 00:11:31.399
of 1929 happens is because
people stopped having confidence

00:11:31.399 --> 00:11:34.905
that stocks are worth as
much as the stock market

00:11:34.905 --> 00:11:35.706
says they are.

00:11:35.706 --> 00:11:37.618
So, everyone pulls out.

00:11:37.618 --> 00:11:40.048
There's a panic, and global banking

00:11:40.048 --> 00:11:41.999
pretty much collapses.

00:11:41.999 --> 00:11:44.704
And that's a really hard
thing to deal with, right?

00:11:44.704 --> 00:11:48.883
I mean it's not like
you're taking your money

00:11:48.883 --> 00:11:51.556
out of the bank or me taking
my money out of the bank

00:11:51.556 --> 00:11:55.617
at any one time could cause
an international depression.

00:11:55.617 --> 00:11:56.058
- [Voiceover] Right.

00:11:56.058 --> 00:11:58.659
- [Voiceover] But when
there is a large group

00:11:58.659 --> 00:12:01.062
of people who all get
panicked at the same time

00:12:01.062 --> 00:12:02.606
and take their money out of the banks,

00:12:02.606 --> 00:12:03.883
the banks fail.

00:12:03.883 --> 00:12:05.579
- [Voiceover] Right, and so
what I think Roosevelt is saying

00:12:05.579 --> 00:12:09.062
is that we cannot allow
a sweeping wave of panic

00:12:09.062 --> 00:12:10.868
to come over the nation again.

00:12:10.868 --> 00:12:11.906
- [Voiceover] Exactly.

00:12:11.906 --> 00:12:14.263
- [Voiceover] So, that's
the context for this speech

00:12:14.263 --> 00:12:15.702
is things are bad.

00:12:15.702 --> 00:12:17.920
The reason things are bad
is because of this wave

00:12:17.920 --> 00:12:21.287
of nameless, unreasoning,
unjustified terror,

00:12:21.287 --> 00:12:23.562
and "America, I need your support

00:12:23.562 --> 00:12:25.583
"to make sure we don't
let that happen again,

00:12:25.583 --> 00:12:27.904
"so we can turn this
retreat into an advance."

00:12:27.904 --> 00:12:29.565
- [Voiceover] In our
next video, we'll go more

00:12:29.565 --> 00:12:32.467
into how we can analyze this source

00:12:32.467 --> 00:12:34.974
and use it to construct
an argument of our own.


2015 AP Physics 1 free response 1c

WEBVTT
Kind: captions
Language: en

00:00:00.329 --> 00:00:02.128
- [Voiceover] Let's now tackle part C.

00:00:02.128 --> 00:00:04.736
So they tell us block 3 of mass m sub 3,

00:00:04.736 --> 00:00:06.030
so that's right over here, is added

00:00:06.030 --> 00:00:07.805
to the system as shown below.

00:00:07.805 --> 00:00:11.522
There is no friction between
block 3 and the table.

00:00:11.522 --> 00:00:14.087
Alright, indicate whether the magnitude

00:00:14.087 --> 00:00:15.946
of the acceleration of block 2

00:00:15.946 --> 00:00:18.407
is now larger, smaller, or the same

00:00:18.407 --> 00:00:20.926
as in the original two-block system.

00:00:20.926 --> 00:00:23.992
Explain how you arrived at your answer.

00:00:23.992 --> 00:00:25.980
So let's just think
about the intuition here.

00:00:25.980 --> 00:00:27.827
If you think about the net forces

00:00:27.827 --> 00:00:31.817
on the system itself, they're the same

00:00:31.817 --> 00:00:34.927
as we had before, now the internal forces

00:00:34.927 --> 00:00:36.321
are going to be different, you're actually

00:00:36.321 --> 00:00:38.190
going to have two different tensions now,

00:00:38.190 --> 00:00:40.059
now that you have two different strings

00:00:40.059 --> 00:00:42.477
but the net forces, the ones that are

00:00:42.477 --> 00:00:46.074
causing this thing to accelerate

00:00:46.074 --> 00:00:48.534
in the upward direction
on the left-hand side

00:00:48.534 --> 00:00:51.545
to the right on the top and then downwards

00:00:51.545 --> 00:00:54.151
on the right-hand side,
it's still this though

00:00:54.151 --> 00:00:56.505
the difference in the weights between

00:00:56.505 --> 00:00:59.813
the two blocks but now
that difference in those,

00:00:59.813 --> 00:01:01.771
inbetween the weights of the two blocks

00:01:01.771 --> 00:01:06.115
is moving more mass and we know that force

00:01:06.115 --> 00:01:08.855
is equal to mass times acceleration

00:01:08.855 --> 00:01:10.899
or acceleration is equal to force

00:01:10.899 --> 00:01:15.136
divided by mass and our
force, our net force

00:01:15.136 --> 00:01:18.098
is being, is the differential
between the weights

00:01:18.098 --> 00:01:19.571
or the difference between the weights

00:01:19.571 --> 00:01:21.687
of the block but now we're going

00:01:21.687 --> 00:01:23.980
to be moving more aggregate mass,

00:01:23.980 --> 00:01:28.194
this is going to be m1 plus m2 plus m3

00:01:28.194 --> 00:01:31.911
and so you're going to have
a smaller acceleration.

00:01:31.911 --> 00:01:36.911
And so what you could
write is acceleration,

00:01:36.913 --> 00:01:40.453
acceleration smaller

00:01:43.013 --> 00:01:48.006
because same difference,

00:01:50.866 --> 00:01:54.131
difference in weights,

00:01:54.131 --> 00:01:56.721
in weights,

00:01:56.721 --> 00:01:59.511
between m1 and m2

00:01:59.511 --> 00:02:03.199
is now accelerating more mass,

00:02:04.469 --> 00:02:09.449
accelerating more mass.

00:02:11.639 --> 00:02:13.985
And that's the intuitive
explanation for it

00:02:13.985 --> 00:02:15.819
and if you wanted to
dig a little bit deeper

00:02:15.819 --> 00:02:17.340
you could actually set up free-body

00:02:17.340 --> 00:02:20.799
diagrams for all of these blocks over here

00:02:20.799 --> 00:02:23.587
and you would come to
that same conclusion.

00:02:23.587 --> 00:02:27.221
So let's just do that, just
to feel good about ourselves.

00:02:27.221 --> 00:02:30.348
So what are, on mass 1

00:02:30.348 --> 00:02:32.665
what are going to be the forces?

00:02:32.665 --> 00:02:37.018
Well you're going to have
the force of gravity,

00:02:37.018 --> 00:02:40.583
which is m1g, then you're going to have

00:02:40.583 --> 00:02:44.565
the upward tension pulling upwards

00:02:44.565 --> 00:02:47.027
and it's going to be larger
than the force of gravity,

00:02:47.027 --> 00:02:48.966
we'll do that in a different color,

00:02:48.966 --> 00:02:52.969
so you're going to have, whoops,

00:02:52.969 --> 00:02:55.131
let me do it, alright so you're going

00:02:55.131 --> 00:02:58.230
to have this tension, let's call that T1,

00:02:58.230 --> 00:03:00.387
you're now going to have
two different tensions here

00:03:00.387 --> 00:03:01.703
because you have two different strings.

00:03:01.703 --> 00:03:04.791
Now the tension there is T1, the tension

00:03:04.791 --> 00:03:07.253
over here is also going to be T1

00:03:07.253 --> 00:03:11.003
so I'm going to do the same magnitude, T1.

00:03:11.003 --> 00:03:12.767
Now since block 2 is a larger weight

00:03:12.767 --> 00:03:15.601
than block 1 because it has a larger mass,

00:03:15.601 --> 00:03:17.248
we know that the whole system is going

00:03:17.248 --> 00:03:21.012
to accelerate, is going to accelerate

00:03:21.012 --> 00:03:22.713
on the right-hand side it's
going to accelerate down,

00:03:22.713 --> 00:03:24.400
on the left-hand side it's
going to accelerate up

00:03:24.400 --> 00:03:27.036
and on top it's going to
accelerate to the right.

00:03:27.036 --> 00:03:29.961
And so if the top is
accelerating to the right

00:03:29.961 --> 00:03:31.720
then the tension in this second string

00:03:31.720 --> 00:03:34.628
is going to be larger than the
tension in the first string

00:03:34.628 --> 00:03:37.830
so we do that in another color.

00:03:41.710 --> 00:03:43.963
I'm having trouble drawing straight lines,

00:03:43.963 --> 00:03:48.026
alright so that we could call T2,

00:03:48.026 --> 00:03:52.231
and if that is T2 then

00:03:52.231 --> 00:03:55.711
the tension through,

00:03:55.711 --> 00:03:58.988
so then this is going to be T2 as well

00:03:58.988 --> 00:04:00.647
because the tension through, the magnitude

00:04:00.647 --> 00:04:02.167
of the tension through the entire string

00:04:02.167 --> 00:04:04.443
is going to be the same, and then finally

00:04:04.443 --> 00:04:07.903
we have the weight of the block,

00:04:07.903 --> 00:04:11.049
we have the weight of block 2,

00:04:11.049 --> 00:04:13.701
which is going to be
larger than this tension

00:04:13.701 --> 00:04:16.123
so that is m2g.

00:04:16.123 --> 00:04:17.991
Now I've just drawn all
of the forces that are

00:04:17.991 --> 00:04:20.967
relevant to the magnitude
of the acceleration.

00:04:20.967 --> 00:04:22.487
If I wanted to make a complete

00:04:22.487 --> 00:04:24.658
I guess you could say free-body diagram

00:04:24.658 --> 00:04:29.658
where I'm focusing on m1, m3 and m2,

00:04:29.882 --> 00:04:32.540
there are some more forces acting on m3.

00:04:32.540 --> 00:04:35.072
M3 in the vertical direction,

00:04:35.072 --> 00:04:39.624
you have its weight,
which we could call m3g

00:04:39.624 --> 00:04:42.502
but it's not accelerating downwards

00:04:42.502 --> 00:04:45.068
because the table is exerting force

00:04:45.068 --> 00:04:47.962
on it on an upwards, it's exerting

00:04:47.962 --> 00:04:50.641
an upwards force on it so

00:04:50.641 --> 00:04:54.124
of the same magnitude
offsetting its weight.

00:04:54.124 --> 00:04:56.399
So that's if you wanted to do a more

00:04:56.399 --> 00:04:58.478
complete free-body diagram for it

00:04:58.478 --> 00:05:00.475
but we care about the things that are

00:05:00.475 --> 00:05:02.355
moving in the direction of the accleration

00:05:02.355 --> 00:05:04.840
depending on where we are on the table

00:05:04.840 --> 00:05:08.114
and so we can just use Newton's second law

00:05:08.114 --> 00:05:10.924
like we've used before,
saying the net forces

00:05:10.924 --> 00:05:13.664
in a given direction are equal to the mass

00:05:13.664 --> 00:05:15.537
times the magnitude of the accleration

00:05:15.537 --> 00:05:16.879
in that given direction, so the magnitude

00:05:16.879 --> 00:05:18.069
on that force is equal to mass

00:05:18.069 --> 00:05:20.110
times the magnitude of the acceleration.

00:05:20.110 --> 00:05:22.522
And so we can do that first with block 1,

00:05:22.522 --> 00:05:23.776
so block 1, actually I'm just going to

00:05:23.776 --> 00:05:25.745
do this with specific, so block 1

00:05:25.745 --> 00:05:27.703
I'll do it with this orange color.

00:05:27.703 --> 00:05:30.339
So block 1, what's the net forces?

00:05:30.339 --> 00:05:35.339
Well it is T1 minus m1g,

00:05:35.444 --> 00:05:37.522
that's going to be equal
to mass times acceleration

00:05:37.522 --> 00:05:41.697
so it's going to be m1
times the acceleration.

00:05:41.697 --> 00:05:43.977
Now what about block 3?

00:05:43.977 --> 00:05:48.242
Well block 3 we're
accelerating to the right,

00:05:48.242 --> 00:05:50.641
we're going to have T2,

00:05:50.641 --> 00:05:52.708
we're going to do that
in a different color,

00:05:52.708 --> 00:05:56.673
block 3 we are going to have

00:05:56.673 --> 00:06:00.252
T2 minus T1,

00:06:00.252 --> 00:06:04.082
minus T1 is equal to m

00:06:04.082 --> 00:06:06.650
is equal to m3 and the magnitude

00:06:06.650 --> 00:06:08.846
of the acceleration is
going to be the same.

00:06:08.846 --> 00:06:10.564
Here we're accelerating to the right,

00:06:10.564 --> 00:06:11.864
here we're accelerating up,

00:06:11.864 --> 00:06:12.770
here we're accelerating down,

00:06:12.770 --> 00:06:14.244
but the magnitudes are
going to be the same,

00:06:14.244 --> 00:06:18.029
they're all, I can denote
them with this lower-case a.

00:06:18.029 --> 00:06:21.301
And then finally we can
think about block 3.

00:06:21.301 --> 00:06:24.194
We could say that the net forces,

00:06:24.194 --> 00:06:28.516
well that's m2g minus T2,

00:06:28.516 --> 00:06:31.218
that's going against m2g,

00:06:31.218 --> 00:06:36.125
is equal to m2 times its acceleration

00:06:37.005 --> 00:06:39.647
and now if we want to
solve for acceleration,

00:06:39.647 --> 00:06:41.040
and this will be quite convenient,

00:06:41.040 --> 00:06:43.339
we can just add up all
of the left-hand sides

00:06:43.339 --> 00:06:45.019
to get a new left-hand side, and add up

00:06:45.019 --> 00:06:46.880
all the right-hand sides to
get a new right-hand side,

00:06:46.880 --> 00:06:48.916
we can do that algebraically
because they're all

00:06:48.916 --> 00:06:50.317
this is equal to that,
that is equal to that,

00:06:50.317 --> 00:06:51.605
that is equal to that so if you add up

00:06:51.605 --> 00:06:53.776
these and then you add up those,

00:06:53.776 --> 00:06:56.807
well then the sums are going
to be equal to each other.

00:06:56.807 --> 00:06:58.862
And so what are you going to get?

00:06:58.862 --> 00:07:02.960
So if you add up all of this,

00:07:02.960 --> 00:07:05.956
this T1 is going to cancel out
with the subtracting the T1,

00:07:05.956 --> 00:07:09.662
this T2 is going to cancel out
with the subtracting the T2,

00:07:09.662 --> 00:07:12.938
and you're just going
to be left with an m2g,

00:07:12.938 --> 00:07:16.278
m2g minus m1g,

00:07:16.278 --> 00:07:20.848
minus m1g, m2g minus m1g

00:07:20.848 --> 00:07:23.668
is equal to and just for,

00:07:23.668 --> 00:07:24.557
well let me just write it out

00:07:24.557 --> 00:07:27.691
is equal to m1a plus

00:07:27.691 --> 00:07:31.511
m3a plus m2a.

00:07:31.511 --> 00:07:33.982
Well we could of course factor the a out

00:07:33.982 --> 00:07:35.674
and so let me just write this as

00:07:35.674 --> 00:07:40.674
that's equal to a times m1 plus m2

00:07:40.785 --> 00:07:43.345
plus m3, and then we could divide

00:07:43.345 --> 00:07:46.865
both sides by m1 plus m2 plus m3.

00:07:46.865 --> 00:07:48.477
So let's just do that.

00:07:48.477 --> 00:07:53.477
So m1 plus m2 plus m3,

00:07:53.545 --> 00:07:57.982
m1 plus m2 plus m3,

00:07:57.982 --> 00:08:00.686
these cancel out and so this is your,

00:08:00.686 --> 00:08:02.519
the magnitude of your acceleration.

00:08:02.519 --> 00:08:05.419
And notice you have the
same difference in weights

00:08:05.419 --> 00:08:08.492
that's providing the
net force on the system

00:08:08.492 --> 00:08:11.186
but is now accelerating more mass,

00:08:11.186 --> 00:08:12.973
so you could even say, hey look,

00:08:12.973 --> 00:08:15.316
I have more mass here,

00:08:15.316 --> 00:08:18.093
so more mass to accelerate,

00:08:18.093 --> 00:08:20.979
more mass,

00:08:20.979 --> 00:08:24.619
more mass to accelerate
while I have the same

00:08:24.619 --> 00:08:26.499
net force acting on the system,

00:08:26.499 --> 00:08:27.788
we're not talking about
the internal forces,

00:08:27.788 --> 00:08:32.504
those all canceled out when
I added these equations

00:08:32.504 --> 00:08:34.467
and so if you're taking the same net force

00:08:34.467 --> 00:08:35.975
and you're dividing it by more mass

00:08:35.975 --> 00:08:37.717
you're going to have a smaller,

00:08:37.717 --> 00:08:39.512
smaller acceleration.

00:08:39.512 --> 00:08:42.484
Hopefully that all made sense to you.