Hi I would appreciate help understanding this question, the answer is D but I do not quite understand why. It won't be B as if Friction acts in that direction it will go toward the outside of the circle. In regards to the rest of the options, I am lost.
A particle undergoing circular motion is constantly changing velocity (vector quantity) even if its angular speed is constant. Therefore, it has a net acceleration.
This acceleration needs to be directed towards the center at all times.
as time passes the velocity curves inward, this must mean that the acceleration is directed inward.
Therefore, the particle must be feeling a force towards the center, and the only source available for this force is the friction. And so the answer is indeed D.
Related.
If the mass is at rest relative to the disc then relative to the ground it is moving in a circle. To keep it moving in a circle there must be a centripetal force directed towards the centre of the circle $O$. The only source of this centripetal force (indeed, the only horizontal force acting on the mass) is friction.
A centripetal force, in the form of friction, is needed towards O for circular motion. A friction force from P to Q would only be needed to allow the mass to undergoing tangential acceleration without slipping, which would only be the case if the angular speed were not constant (i.e., there was angular acceleration).
Hope this helps.
