Recently I asked a question regarding frictional forces at math stack-exchange(because its basically part of maths syllabus) and I drew some conclusions.
If A and B are in rough contact and are in limiting equilibrium, then there exist two frictional forces. One acting on A and one acting on B.
The direction of frictional forces can be determined by working out the direction of motion if the friction was not present.(This is a trick to work out the direction of frictional force which was mentioned on https://physics.stackexchange.com/a/94837/128083)
The direction of friction forces on A and B are opposite.
1,2 are trivial and were implied on the math stack exchange link. However, I came up with rule 3 by observing the relative velocities of each object.
Let's say A is a wall and B is a ball that is in rough contact with A. From A perspective, B is falling down hence an upward friction force should be acting on B. From B's perspective, A is moving upwards, hence a downward force should act on A.
However the problem arises in the following question
(By downward and upward, I mean upward tangential and downward tangential at point P)
The ball P is moving downwards. An upwards frictional force acts on P. Edit An upward frictional force at P imply a downward frictional force for the disc at P. Since frictional force is opposite the direction of movement, thus a downward friction imply an upward movement and hence a clockwise moment. However, it seems trivial that the disc should rotate anti-clockwise but using the rule 3, we can conclude that motion is clockwise.
Can somebody please explain the fallacies in the proposed rule 3? If there does exist a fallacy please explain some alternative that I can employ in working out the direction of frictional forces.
The link mentioned above is: https://math.stackexchange.com/q/2303592/335742