# The directions of the frictional force acting on the cylinder while ascending and descending cylinder

A cylinder rolls up an inclined plane,reaches some height,and then rolls down without slipping throughout these motions.The directions of the frictional force acting on the cylinder

(A)While ascending the incline

(B)While descending the incline

are?

In my book it is given friction acts up the incline in both cases.I find it difficult to understand.Can someone please explain?

P.S:Explain with a digram if possible.I always had trouble in understand direction of rolling friction.Even my school teacher could'nt explain properly.Please help me.Thanks :-)

While descending, as it is fixed for a moment, so it doesn't have initial inertia. Force $mg\sin\theta$ wants to pull the cylinder downwards the incline. So, the cylinder tends to translate downwards and hence, friction force tries to opposes its translation and is upwards again.