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This is a picture of
the planet Lubricon-VI.

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And Lubricon-VI is a
very special planet

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because it's made up of
a yet to be discovered

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element called Lubrica.

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And Lubrica is special
because if anything glides

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across the surface
of Lubrica, it

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will experience
absolutely no friction.

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So if this right over here
is a sheet of Lubrica--

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we're looking at
it from the side.

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And if we have a brick on top of
it, maybe gliding on top of it

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like that, it experiences
absolutely no friction.

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Now, the other things we
know about Lubricon-VI

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is it's drifting in deep
space and it does not

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have an atmosphere.

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In fact, it is a complete
vacuum outside of it.

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It's in such deep space,
such a remote part of space,

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that there aren't even a few
hydrogen atoms right over here.

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It is a complete,
absolute vacuum.

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And it's also an ancient planet.

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The star that it
used to orbit around

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has long since died away.

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So it's just this lonely
planet drifting in deep space

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without an atmosphere.

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The other thing we
know about Lubricon-VI

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is that it is a perfect sphere.

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It is a perfect, perfect sphere.

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Now, my question to you.

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For some bizarre
reason there happens

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to be, on the surface
of Lubricon-VI--

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so this right over here is
the surface of Lubricon-VI.

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There happens to be a sock that
is frozen in a block of ice.

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So this is my sock and its
frozen in this block of ice.

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And it happens to be traveling
at 1 kilometer per hour

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in that direction.

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If we were to look at it
from this kind of macro scale

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when we're looking
at the planet,

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let's say then that
is the frozen sock,

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and it is traveling
along the equator.

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It is traveling along the
equator of Lubricon-VI.

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So my question to you,
given all of the assumptions

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we made that it has
absolutely no atmosphere,

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it's a perfect
sphere, and Lubrica

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has absolutely no
friction regardless

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of what's traveling
on top of it-- what

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will happen to this
frozen sock over time?

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To answer that question,
we need to think about all

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of the forces that
are acting on this,

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I guess, frozen block
of ice and sock.

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And first of all, let's
think about these forces that

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are acting in the radial
direction, inward or outward,

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of the center of the planet.

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Well, this planet has a mass.

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And so you have an inward force
towards the planet's center

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of mass.

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And so you have the
force of gravity

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acting on this
block going radially

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inward to the center
of the planet.

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So I'll draw it like this.

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So we have our force of gravity.

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We have our force of gravity
going radially inward,

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just like that.

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But then we know that
the block is not just

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spiraling towards the
center of the earth.

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We have the surface here.

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It's not going to go through
the surface of Lubrica.

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We can also assume that
Lubrica is a very, very, very

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strong material.

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And so you also
have a normal force.

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You also have a
normal force that

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is keeping the
block from spiraling

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towards the center of the earth.

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So this is a normal force.

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And one thing we'll
think about now,

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and we'll address it
directly in another tutorial,

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is whether this normal force is
equal to the force of gravity.

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We'll think about that
in a future video.

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But these are all
the forces that

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are acting in the
radial direction,

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either inward towards the
center of the planet or outward.

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But if we think about in
the tangential direction,

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along the surface of the
planet, there are no net forces.

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And because there
are no net forces

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in this tangential
direction right over here,

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this block will not either
accelerate nor decelerate.

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There is no air friction.

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Or I should say air
resistance, which is really

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just friction with the particles
if you had an atmosphere.

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It's a complete vacuum,
so there's nothing there.

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There is no friction with
the surface of the planet.

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So there's no
friction there, which

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could have been a force in
the tangential direction.

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So there's absolutely no forces
in the tangential direction.

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So this block of
ice will actually

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continue to travel at
one kilometer per hour

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for all of eternity.

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So it'll just continue
to do it given

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the assumptions that
we've just made.

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