Why Does a Rolling Ball Stop Moving but Not Roll?

[Gyroscope]

Homework Statement

I throw a ball rolling and moving through the a flat surface. I may say that friction is slowing down translation motion, but increasing rotational motion, so when the ball stops moving, why is it not rolling? That's confusing me a bit. What can you say?

 

[Doc Al]

Can you rephrase your question. If the ball is at rest (no translational motion or acceleration) how can it be rotating with friction present? (Is that what you mean by rolling?)

 

[Gyroscope]

I am sorry, my english did not let express as well as I would like to. The ball initially has rotational and translational motion.

Thanks for replying.

 

[Doc Al]

OK. If the ball starts with a motion such that the speed of its bottom surface (with respect to the floor) is anything other than zero, then kinetic friction will be exerted on the ball in a direction that opposes the slipping between the surfaces.

For example: If the ball starts out in pure translation, then friction will slow the translation and speed up the rotation--up until the point that the ball rolls without slipping.

 

[Gyroscope]

And if the ball starts out with pure rotation?

Thanks again for helping me.

 

[Doc Al]

And if the ball starts out with pure rotation?

The direction of rotation will determine which way friction will act and the direction that it will roll. This time kinetic friction will speed up the translation and slow down the rotation until the ball rolls without slipping.