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### Course: Physics archive>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? Explore the intriguing world of Lubricon-VI, a fictional planet with zero friction and no atmosphere. This video delves into the forces acting on a frozen sock sliding on its surface. It highlights the absence of tangential forces, ensuring the sock's eternal journey at a constant speed. 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?
• 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.
• 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?
• 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.
• What are radial directions ?
• 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).
• If a ball is bouncing on the surface of Lubricon-VI, will it stop bouncing even though there is no friction?
• 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!)
• What will happen to an object on a planet which has no gravity??
• a) it would not be attracted to the planet

b) any planet (or object) with a mass would 'produce' a gravitational field...
• what is centripital force
? how would it effect is the planet was not a prefect sphere?
• 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).
• How is it even possible for the normal force to be less than the gravitational force?
• 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.
• 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?
• 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.
• Is Lubricon V1 real? or did Sal make the planet up for the purposes of the video?