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

Question

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 :-)

Answer 1

Friction force always opposes relative motion between two surfaces. In most cases (like yours), one of those surfaces is fixed. So, you should recognize that how does the other surface tend to move. For this purpose, you should consider that what force or torque want to move the body. Then, you can determine the correct direction of friction.

While ascending, the cylinder wants to roll because of its initial rotational inertia. So, friction force tries to oppose its rotation and act upwards the incline.

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.

(This answer can help you more!)