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Old Jun 22nd 2013, 03:33 AM   #1
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How will objects roll in the absence of friction?

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?
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Old Jun 22nd 2013, 05:46 AM   #2
MBW
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Effectively the friction free surface can be ignored!
Unless there is a component of force or motion perpendicular to the surface it will have zero effect on the motion of the ball.
Thus in your example the frictition free surface is only counteracting gravity
and the result will be exactly the same as in a zero gravity environment.
as you rightly say the ball will only roll if you apply a moment

Consider a snooker ball, the friction between ball and cloth is purposely kept very low.
By adjusting the striking position of the cue, the spin of the ball can be very different from the natural spin of a rolling ball...
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