# Homework Help: Friction and acceleration

1. Jun 22, 2015

### RingNebula57

We know that when a ball is rolling ( just rolling no slipping) the frictional force is orientated in the direction of motion.
Why isn't the ball accelerating due to frictional force, if it is the only force in the motion direction?

To me it's quite daunting

2. Jun 22, 2015

### PWiz

It is accelerating, its just that the acceleration is negative and opposes motion (commonly known as deceleration).

3. Jun 22, 2015

### CWatters

Do we? You need to be clear what you mean by frictional force. What frictional force is there if the ball is rolling along at a constant velocity?

Perhaps you mean rolling resistance?

4. Jun 23, 2015

### RingNebula57

Ff is the friction force that's making the ball roll. If Ff is the only force acting upon the ball in the horizontal direction and the ball rolles in the Ff direction, than shouldn't the ball accelerate ( not decelarate, but go on and on )

5. Jun 23, 2015

### SammyS

Staff Emeritus
Is the ball on a level horizontal surface?

6. Jun 24, 2015

### RingNebula57

yes

7. Jun 24, 2015

### jbriggs444

If the ball is rolling on a level horizontal surface and is continuing to roll at a steady rate then there is no sense in which we can say that friction is "making it roll". If anything, inertia is "making it roll".

The force of friction is zero and the ball has zero acceleration. This is consistent with Newton's second law. F = ma. 0 = 0.

8. Jun 24, 2015

### RingNebula57

If the force of friction was 0 than why is the ball rolling? Why isn't it just slipping ( like on ice) ?

9. Jun 24, 2015

### PeroK

If a ball is already rolling and it rolls onto ice (smooth, frictionless surface), then it will continue to roll.

If the ball is rolling on a rough surface it is essentially static friction. The point of the ball that touches the ground is not moving (instantaneously at rest), so the frictional force does no work. Just as it would if the ball was not moving.

10. Jun 25, 2015

### CWatters

What PeroK said.

The ball has inertia (eg a moment of inertia).
If it's already rolling when it encounters the ice it will keep rolling (nothing to stop it rolling)
If it's not rolling (eg it's sliding) when it encounters the ice it will keep sliding (nothing to make it start rolling)