This is a picture of the planet Lubricon-VI.
And Lubricon-VI is a very special planet
because it's made up of a yet to be discovered
element called Lubrica.
And Lubrica is special because if anything glides
across the surface of Lubrica, it
will experience absolutely no friction.
So if this right over here is a sheet of Lubrica--
we're looking at it from the side.
And if we have a brick on top of it, maybe gliding on top of it
like that, it experiences absolutely no friction.
Now, the other things we know about Lubricon-VI
is it's drifting in deep space and it does not
have an atmosphere.
In fact, it is a complete vacuum outside of it.
It's in such deep space, such a remote part of space,
that there aren't even a few hydrogen atoms right over here.
It is a complete, absolute vacuum.
And it's also an ancient planet.
The star that it used to orbit around
has long since died away.
So it's just this lonely planet drifting in deep space
without an atmosphere.
The other thing we know about Lubricon-VI
is that it is a perfect sphere.
It is a perfect, perfect sphere.
Now, my question to you.
For some bizarre reason there happens
to be, on the surface of Lubricon-VI--
so this right over here is the surface of Lubricon-VI.
There happens to be a sock that is frozen in a block of ice.
So this is my sock and its frozen in this block of ice.
And it happens to be traveling at 1 kilometer per hour
in that direction.
If we were to look at it from this kind of macro scale
when we're looking at the planet,
let's say then that is the frozen sock,
and it is traveling along the equator.
It is traveling along the equator of Lubricon-VI.
So my question to you, given all of the assumptions
we made that it has absolutely no atmosphere,
it's a perfect sphere, and Lubrica
has absolutely no friction regardless
of what's traveling on top of it-- what
will happen to this frozen sock over time?
To answer that question, we need to think about all
of the forces that are acting on this,
I guess, frozen block of ice and sock.
And first of all, let's think about these forces that
are acting in the radial direction, inward or outward,
of the center of the planet.
Well, this planet has a mass.
And so you have an inward force towards the planet's center
of mass.
And so you have the force of gravity
acting on this block going radially
inward to the center of the planet.
So I'll draw it like this.
So we have our force of gravity.
We have our force of gravity going radially inward,
just like that.
But then we know that the block is not just
spiraling towards the center of the earth.
We have the surface here.
It's not going to go through the surface of Lubrica.
We can also assume that Lubrica is a very, very, very
strong material.
And so you also have a normal force.
You also have a normal force that
is keeping the block from spiraling
towards the center of the earth.
So this is a normal force.
And one thing we'll think about now,
and we'll address it directly in another tutorial,
is whether this normal force is equal to the force of gravity.
We'll think about that in a future video.
But these are all the forces that
are acting in the radial direction,
either inward towards the center of the planet or outward.
But if we think about in the tangential direction,
along the surface of the planet, there are no net forces.
And because there are no net forces
in this tangential direction right over here,
this block will not either accelerate nor decelerate.
There is no air friction.
Or I should say air resistance, which is really
just friction with the particles if you had an atmosphere.
It's a complete vacuum, so there's nothing there.
There is no friction with the surface of the planet.
So there's no friction there, which
could have been a force in the tangential direction.
So there's absolutely no forces in the tangential direction.
So this block of ice will actually
continue to travel at one kilometer per hour
for all of eternity.
So it'll just continue to do it given
the assumptions that we've just made.