# I How can objects even roll?

1. Oct 4, 2016

### FallenApple

So in most of intro physics rolling questions, the masses are assumed to be rigid. So if I roll a sphere on frictional plane, it will move forward and the friction force will help stablize it until pure roll.

However, how can there even be a friction force assumed in these problems? The point of contact is one point of dimension 0. How can a friction force be exterted on this point? Also, even if it could be pushed. There is no point paralell to it to push it as all the points in the plane are constantly below it.

2. Oct 4, 2016

### PeroK

Theoretically a force can be applied at a single point. In practice, there will always be a small area of contact between the object and the surface.

3. Oct 4, 2016

### Drakkith

Staff Emeritus
Are you saying the pushing force is applied by a point on the previously mentioned plane?

4. Oct 4, 2016

### FallenApple

What
What I mean is that the ground can only exert a normal force up. It can't exert a force to the side. In sliding friction problems, the only way we can assume friction works is the microspicly, the surfaces are jagged.

I don't see how a side way force can be produced for rolling on a point. Unless, if we zoom in, finding the ball is more like a gear.

5. Oct 4, 2016

### FallenApple

Yes. The point that applies the friction is the point in contact with the bottom of the ball.

The point on the bottom ball is always directly above, with virtually 0 distance, the point of contact on the ground. So unless they are like gears, there can't be any locking to change rotations etc.

6. Oct 4, 2016

### A.T.

We don't care how friction works on the microscopic level, in this kind of problem. We just use empirically derived macroscopic models.

7. Oct 4, 2016

### PeroK

Again, theoretically a surface with friction can exert a tangential as well as a normal force. If you are asking about the nature of friction, then it must result from the sort of interaction you suggest. A snooker ball must be deforming the cloth to some extent.

That doesn't invalidate the simplified model, as long as the model produces accurate results.

8. Oct 5, 2016

### CWatters

As the contact area approaches zero the pressure increases towards infinity. So it's very hard to have a perfect zero area point contact. For example you need infinitely hard surfaces to avoid them distorting.

In short it's hard to avoid all friction in the situation you describe.