# Friction on on circular objects while rolling?

1. Aug 9, 2011

### prashantjha

friction on on circular objects while rolling??

i had a doubt about circular rolling objects..
it is true that friction opposes relative motion..but while doing so in a rolling wheel rolling forward,the friction also works in forward direction.so then my question is why does the wheel stops after a particular time.if friction is helping it move,then it should move till infinity if left undisturbed??

2. Aug 9, 2011

### Bloodthunder

Re: friction on on circular objects while rolling??

friction is not helping it move, friction is helping it roll. without friction, the ball is unable to roll at all and will just slip down the slope.

also, the friction on the ball is not in the direction of movement but in the opposite direction, it is the friction on the ramp by the ball that is in the ball's direction of movement. so the friction on the ball is still going in the opposite direction and eventually stops the ball.

3. Aug 9, 2011

### mikeph

Re: friction on on circular objects while rolling??

Friction is not helping it roll in any sense. If you magically removed friction halfway through the experiment, conservation of angular and linear momentum would mean the ball continues in exactly the same way as before (with the exception that it never slows down).

Friction initially influences the rolling motion by enforcing the non-slip condition v = rw. After this, it doesn't do anything but slow the ball down.

4. Aug 9, 2011

### rcgldr

Re: friction on on circular objects while rolling??

Rolling resistance opposes relative motion. This is due to the losses in deformation (compression) and restoration (expansion back to original shape) of the surfaces at and near the contact point.

Static friction opposes external forces.

Kinetic (sliding) friction opposes relative motion between surfaces, but in this case, an object is not truly rolling any more.

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