rolling & slipping

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?

please help me out. i am tired of reading so many books. still not able to solve numerical related to it

A.T.

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

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.

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?

Doc Al

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