Q.

A solid cylinder is rolling down a rough inclined plane of inclination θ. Then                        [2006]

1 The friction force is dissipative  
2 The friction force is necessarily changing  
3 The friction force will aid rotation but hinder translation  
4 The friction force is reduced if θ is reduced  

Ans.

(3, 4)

The sliding friction acts in the opposite direction of mgsinθ to oppose the relative motion. Because of frictional force the cylinder rolls.

Hence frictional force aids rotation but hinders translational motion.

Applying Fnet=ma along the direction of inclined plane, mgsinθ-f=mac,

where ac is acceleration of centre of mass of the cylinder

 f=mgsinθ-mac                                     ...(i)

But ac=gsinθ1+IcmR2=gsinθ1+mR2/2mR2=23gsinθ

 f=mgsinθ3

Clearly, if θ is reduced, frictional force is reduced.