Friction

Basic definition of friction is that it opposes the relative motion b/w two surfaces.

i hope u can understand the meaning of zero work done by friction urself from the above definition.!

Work done by friction is negative when it acts opposite to the direction of motion of the object. eg: when a body slides along an inclined plane.

Work done by friction is positive when it causes the motion of an object. eg : the force acting on wheels of a bicycle. when u pedal a bicycle, the tires apply a backward force on the road, as a counter to this force, frictional force acts in the forward direction on the tire/wheel. which is responsible for it's motion.

try focussing on the work done by friction everytime when u solve a mechanics problem. This will strengthen ur concept. :)

please answer !

@sparkle,

1) friction acts backwards. + no work done by friction.
as it is pure rolling [it is not slipping, u told] so, the ppoint of contact
is stationary .. thus, though there is friction, but as there is no
component of displacement parallel to frcition, so work done by friction =0.

2.) second bhi pure roling ka case hai and agar na bhi ho toh... friction kaha se aega!
surface is smooth [3] ... so work done by friction is zero.

3.) friction acts in the backwards direction. [why -> the ball is projected with a forward linear velocity.. so the point of contact has a forward linear velocity. Now, if the body has to attain pure rolling, it has to have v in the backward direction. This it can attain by rotating and friction 'helps' in this rotatiing. It initiates a clockwise torque...]
now, since friction is in backward direction and displacement of point of contact is in forward direction ; so work done by friction = -ve.

4.) u try this one yourself :)