# Rolling & slipping

1. Jan 11, 2013

### dev70

hi pf, the more i think i have understood classical rotational mechanics the more it turns out to be clumsy? i am just not able to understand the slipping and rolling of bodies? somewhere friction comes into play and somewhere dont?
"if a body rolls without slipping such that velocity of COM doesnt change then no frictional force acts on the body" how? why would a body roll if there is no friction?

2. Jan 11, 2013

### A.T.

It's not about how it started rolling. Once it is rolling at constant speed no friction is necessary.

3. Jan 11, 2013

### CWatters

What AT said.

It actually says that in the quote...

Consider a train that has solid steel wheels (so negligible rolling resistance).... When it accelerates it applies a force on the track in one direction, when it brakes it applies a force in the opposite direction. When it does neither and travels at a constant velocity no frictional forces are acting.

Aside: In a real train this isn't true because of things like wind resistance and rolling resistance isn't totally zero. So some force (and power from the engine) is required to maintain a constant velocity.

4. Jan 11, 2013

### dev70

ok..if i consider a sphere rolling as well as sliding down an inclined plane? what should i interpret? friction is acting or not?

5. Jan 11, 2013

### Staff: Mentor

If the sphere is rolling without slipping down the incline then there must be a friction force to produce the angular acceleration.