Hey Crazies.
The other day, Question Clone was asking me something:
Why does the Earth keep spinning?
The answer is Inertia.
An object’s ability to resist changes in motion.
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Wait a minute!
Newton’s first law only applies to steady speed in a straight line.
Gah! You got me!
OK, so just saying “inertia” is a bit dismissive of this question.
We need a deeper look.
If you take an introductory physics class, they’re probably going to tell you something like this:
Moving from one place to another involves a set of measurements,
which are governed by certain rules.
Going around and around involves a different set of measurements,
governed by a different set of rules.
But I said something very important in a previous video:
If your rule has exceptions, then it’s not that great of a rule.
The goal of physics to find better rules that have fewer exceptions
and we do a pretty good job.
These rules actually apply to both types of motion.
They’re just not very convenient the way they’re written.
Let’s start with something deceptively simple.
That’s just a ball!
No one asked you!
Most of you look at this and see what my clone sees a tennis ball.
But it’s so much more than that.
This ball is made of a rubber shell
covered in nylon felt
surrounding a pocket of air.
Each of those materials is made of molecules, which are made of atoms,
which are made subatomic particles, some of which are made of even smaller particles,
which are just excitations in quantum fields, held together by particle interactions.
All just to make this thing you call a tennis ball.
OK, that was a little deep.
My point is that everything is like this, including the Earth.
If you want to understand how these rules apply to a rotating object,
then you need to look at an object as a collection of parts rather than a whole.
The Earth has several layers made of a bunch of different substances,
and, because of inertia, all of the little
parts are trying to move in straight lines.
If they were allowed to, the Earth would do this.
We know it isn’t though, so there must be something else going on.
Newton’s first law says that anything will move at a steady speed in a straight line
until there’s a total or unbalanced force
from the outside.
If the parts of the Earth aren’t doing this, there must be a force preventing it.
That’s where Newton’s second law tells us that unbalanced force is causing an acceleration.
Acceleration? But its speed is steady: 1 rotation every 24 hours.
Ah! But acceleration isn’t just about change in speed.
“Acceleration” is a change in “velocity” which involves direction too.
A car accelerates forward because the force from the road is in the same direction
as the motion of the car, increasing its speed.
The force on a thrown ball is only partly in the direction of motion,
which causes it to slow down and speed up, all the while changing its direction too.
If the force is never along the direction
of motion, then you get circular motion.
The speed never goes up or down.
The force only changes the direction.
The same thing is happening to the parts of the Earth while it spins.
But it’s still an acceleration.
And it’s caused by subatomic particle interactions, intermolecular forces
and, on the largest scale, gravity.
But, if you’re trying to describe the motion of a ball or the Earth,
you don’t always want to consider the momentum and energy of its parts
or the forces between them.
Sometimes, you just want to say this is a ball.
So we ignore all the forces that only change direction
and just call that “rotational inertia.”
Any forces that change the speed, we multiply by a distance and call that a torque.
You get something that still looks and works a lot like Newton’s second law.
When you’re trying to be practical, sometimes it’s easier just to pretend and use different rules.
There are a lot of science questions like this that teachers just dismiss away.
Got any that drive you crazy?
Ask in the comments.
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And until next time, remember, it’s OK to be a little crazy.
