If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

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?

  • leaf blue style avatar for user Paolo Miguel Bartolo
    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)
    Default Khan Academy avatar avatar for user
  • leafers ultimate style avatar for user Jørgen Pedersen
    At you 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)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Charles LaCour
      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)
  • spunky sam blue style avatar for user Chunmun
    What are radial directions ?
    (9 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Martín Torres
    If a ball is bouncing on the surface of Lubricon-VI, will it stop bouncing even though there is no friction?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user echo wang
      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)
  • purple pi teal style avatar for user leenavaishali
    What will happen to an object on a planet which has no gravity??
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leaf red style avatar for user kushagra
    what is centripital force
    ? how would it effect is the planet was not a prefect sphere?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user Ramana
    How is it even possible for the normal force to be less than the gravitational force?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • female robot grace style avatar for user Maddie
    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)
    Default Khan Academy avatar avatar for user
    • starky seed style avatar for user Quinn Ciccoretti
      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)
  • winston baby style avatar for user abnerkahan
    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)
    Default Khan Academy avatar avatar for user
  • primosaur ultimate style avatar for user Sukhayl Niyazov
    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)
    Default Khan Academy avatar avatar for user

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.