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## Physics library

### Course: Physics library > Unit 3

Lesson 4: Slow sock on Lubricon VI# Slow sock on Lubricon VI

What would happen to a slowly moving frozen sock on a frictionless planet. Created by Sal Khan.

## Want to join the conversation?

- What's the significance of assuming that Lubricon-VI is a perfect sphere? What will happen if the planet is not a perfect sphere?(27 votes)
- If it is not a perfect sphere you would have to include gravitational potential energy into the equations because the distance from the planets center of mass would change.(60 votes)

- At3:22you draw the normal force acting on the block. I am used to letting the force vectors start from the point where they are acting on an object. The force of gravity acts from the center of the earth on the center of the block of ice, that's ok. But the normal force is acting on the block on the lowest point, the place nearest the ground. Why do you draw the normal force starting from the center of the object? Wouldn't it be better for the sake of the aspiring physicist to let the force vectors start at the impact point, explaining the cause of the force?(12 votes)
- In a real life situation you would calulate things based on where the force is acting. If this force is not directed through the objects center of mass you will introduce a rotational force into the system. So In idealised physics problems it is a convention to assume that the force is acting through the center of mass.(19 votes)

- What are radial directions ?(9 votes)
- Radial direction is the dimension which points away from the origin. The angular direction is the dimension which rotates around the origin. This is the basis for polar coordinates (r,θ) vs Cartesian (x,y).(14 votes)

- If a ball is bouncing on the surface of Lubricon-VI, will it stop bouncing even though there is no friction?(3 votes)
- If the ball collides with the surface of the planet, according to the law of conservation of energy, the gravitational potential energy of the ball is partly converted into energy of motion. If it is not an elastic collision, then the collision will cause energy loss, and the ball will slowly stop. If it is an elastic collision, then there will be normal force between them, which will cause the ball to do work(W=delta E) on the ground, leading to energy loss and eventually being stationary on the ground.(sorry for my poor English.Hope you could understand me!)(8 votes)

- What will happen to an object on a planet which has no gravity??(2 votes)
- a) it would not be attracted to the planet

b) any planet (or object) with a mass would 'produce' a gravitational field...(9 votes)

- what is centripital force

? how would it effect is the planet was not a prefect sphere?(4 votes)- Centripetal force is any force which keeps a body in orbit around a central body. Your second question is answered above by Teacher Mackenzie(UK).(2 votes)

- How is it even possible for the normal force to be less than the gravitational force?(2 votes)
- If you get in an elevator and it accelerates downward, you will feel lighter. That's because the normal force on you is less than your weight.(5 votes)

- Does the object have to be traveling around the equator of the planet? Would anything change if, for instance, the sock was traveling in a smaller circle farther from the equator?(3 votes)
- In this example, yes. Let's say you were to do something similar to the video around say, the tropic of capricorn.

Now lets say our frame of reference moves in a circle in such a way that the y direction always points "Up," that is away from and perpendicular to the surface. In this spinning reference frame, it is clear as in the video that there is no side to side motion, all we have is a normal force and gravity, so it will not deflect side to side.

And since F(gravity) = F(normal), there is no net acceleration in any direction, so it will stably go around forever.

But if we were to remove the normal force and give enough velocity for this sock to orbit without touching the ground, it would not be stable, things can only orbit stably in equators (or ellipses) that go all the way around the circumference.(3 votes)

- A:Will gravity slow the sock down?

B: How is it possible to apply a force in a friction-less area? One needs a friction as leverage to apply a force?(3 votes)- a: Not on Lubricon

b: Gravitational interaction is not transmitted by friction. Neither is electric force(2 votes)

- Why there are no net forces if it moves with the speed of 1 km per hour? From where does this force which moves at the speed of 1 km per hour derives?(2 votes)
- Momentum maintains its motion at a speed of 1 km/h. The lack of a net force keeps it from accelerating.(2 votes)

## Video transcript

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.