# How will objects roll in the absence of friction?

1. Jun 22, 2013

### physicsjn

I am just wondering (out of curiosity) these past few days what would happen if an object rolls on a frictionless surface.

I was thinking if a spherical object is on a free space and is subjected to some force field, I think it would just move laterally to the direction of the field and would not roll, isn't it?

But if that object is on a horizontal frictionless surface and is subjected to some force field, will it roll or just move laterally like what it would do in free space?

If it rolls, then how does it roll (I am thinking about the mechanics of its rolling)? I was thinking that for objects to rotate or roll, it should have net torque. In free space, the tangential force on the top and bottom part of the spherical object points in the same direction (but moment arms are opposite in direction), hence the torque should cancel. On the surface, the applied force on the top part (which is also acting on the bottom part) and the frictional force at the bottom are opposite in direction, hence their torque should add up. I think it is the frictional force that produces the rolling motion on the surface as opposed to free space.

But in the absence of friction, how can objects roll on a surface? If equal forces are acting on the top and bottom part of the sphere, wouldn't the torque cancel, causing the object to have only lateral motion?

Thank you for reading my question.

2. Jun 22, 2013

### Drakkith

Staff Emeritus
You are correct. It would not roll in the absence of friction.

3. Jun 22, 2013

### I like Serena

In the absence of torque, angular momentum is conserved.
In other words, an object would not start rolling, so it would just slide over your surface without rotating.

And if it was already rotating (or rolling), it would remain rotating with the same rotation speed.
So it would be sliding over your surface and rotating independently, with or without slipping, but either way without changing its angular velocity.

4. Jun 22, 2013

### physicsjn

Hmm.. I see; thank you very much. I was thinking of this and I can't imagine a spherical ball moving just laterally on the surface without rolling (probably because I've never seen frictionless surface in real life.) I also haven't seen any videos. However, my reasoning tells me that it can happen, so I got bothered and can't concentrate on my problem sets. Thank you very much. :D

5. Jun 22, 2013

### I like Serena

Try it with a really slippery surface, say black ice, and a massive ball (high on inertia).
You'll see. :)

Or perhaps just try bicycling on black ice (I did without intending to).
The wheels have no purchase at all.
Rolling or going straight loose their meaning.
Do to take proper safety precautions because it hurts!

6. Jun 22, 2013

### dreamLord

You can see slipping of a spherical object initially while bowling. After some time it starts rolling without slipping.