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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 7: Slow sock on Lubricon VI

# Normal forces on Lubricon VI

Explore the intriguing world of Lubricon VI, a fictional non-rotating planet, and its unique physics! Understand the forces acting on a stationary frozen banana and a perpetually orbiting frozen sock. Delve into the concepts of gravity, normal force, and centripetal acceleration in this engaging thought experiment. Created by Sal Khan.

## Want to join the conversation?

• Isn't the force of gravity equal to the force normal in both cases?

Let's say that the moving block did have the same magnitude for both forces in the y direction, if it's true that the block would just leave the surface, then the block would no longer have a normal force. So wouldn't the force of gravity pull it back down to the surface?

I agree that the object does not have a net force of 0 because the initial force push (the one that set it into motion) is continuing to act on the object due to the frictionless nature of lubricon.

But why does the force normal not equal the force of gravity?
• You're correct in thinking the block would no longer have a normal force and gravity would pull it back down if it left the surface. But the block doesn't have to entirely leave the surface for the normal force to decrease. It can decrease at a very small scale since the surface is slightly curved and the block keeps wanting to continue in a straight path and leave the surface. It just isn't quite as dramatic as the block flying off the surface and being pulled back down, but more subtle like the feeling you get when driving a car over a rolling hill. In that case, the normal force on you decreases, and there is a net force pulling you down the hill and you can feel it in your stomach. The net force on the block is at a much smaller scale than this because the curvature is that of the entire planet (and not just a hill).

As far as the initial force goes, after the force has been applied to get the object moving, it goes away. Afterwards, the only forces involved are from gravity and the normal force.
• At , he says that the banana has no tangential velocity, and that the sock will retain it's velocity (which, actually, should be speed, since its direction changes as it orbits). Would the banana and the sock not have a gravitational effect on each other, causing them both to accelerate towards each other?
• They would have an acceleration of of G*m1*m2/d^2/m1 and G*m1*m2/d^2/m2 or a total speed towards each other of (G*m1*m2/d^2/m1 + G*m1*m2/d^2/m2) * t
G = gravitational constant = 6.67*10^-11

This means that if the distance between the two objects was 1 km (d = 1000), m1 = 1 kg and m2 = 2 kg, the speed after 10^8 years would be 0.421m/s

I might be totally off though :)
• Why can't a non-rotating planet have an equator?
• The equator is defined to be perpendicular to the axis of the planet. If the planet is not spinning, then there is no axis. If a planet does not have an axis, then we cannot define the equator either.
• If the planet were rotating, what effect would that have on either block? Would it make a difference if the sock were going around the equator or the poles?
• No effect, since there is no friction. However, there is the problem of the relative speed. When we say moving at one km per hour in the East, is that relative to the planet's surface or to an observer in space? If it were relative to the surface, and the planet was rotating at 2 km per hour to the East (measured at the surface), then the object would be moving 3 km per hour to the East.

tldr:
No effect
• I"m completely confused. The sock is traveling at a constant velocity, not accelerating. Why is there any tangential force?
• The sock's speed is constant, but it's velocity isn't.
Yes, it's moving at 1km/h, but it's direction is constantly changing. For the sock to circle around the planet (which, in this case, would be the same as orbit around it), it needs a centripetal acceleration curving it's movement inwards, keeping it from flying away.
If it's velocity were constant, the sock would move tangentially away from Lubricon VI and into deep space.
• How is there gravity? Doesn't the planet need to be rotating inorder to be able to have a gravitaional force? Or is this just theoretical gravity?
• The planet doesn't have to be rotating to have gravitational force.Gravitational force is the force between any 2 objects in the universe.Anything in this universe that has mass, would bend space, which results in gravity.Gravity acts between any 2 bodies in the universe (including you and me, but its magnitude would be very very small).So, a planet doesn't have to be rotating to have gravity, as long as it has mass, it would have gravity
• If the frozen sock block's movement comes Fg being greater than Fn, why does the sock block move around the planet instead of sinking into the center of the planet (the direction of the net force unbalance.
• The movement of the sock is not caused by the force Fg.
That motion has been caused by some other means that we have not been told about. We do know that it was a force with a component TANGENTIAL to the surface of the planet.
The sock is now travelling with constant speed around the friction-less planet

He is saying that Fg must be greater than Fn because there is a centripetal acceleration (that the banana does NOT have) simply keeping the sock moving in a circular path and nothing else and that Fg is providing this centripetal force

Hope that helps

IM
• Is the force of gravity 'round'? That is, would an object with mass, hence gravity, get formed into a sphere? Why not a square planet? I assume the roundness is due to rotation, and a non rotating planet might lose its roundness....