# Frictional forces and their directions

Recently I asked a question regarding frictional forces at math stack-exchange(because its basically part of maths syllabus) and I drew some conclusions.

1. 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.

2. 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)

3. 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: Frictional forces

• I have posted this question on physics-stackexchange but the question has remained unanswered for a while. If it is against the policy to upload similar question on different stackexchanges let me know. – mathnoob123 May 31 '17 at 17:46
• Your conclusions 1 and 2 are both wrong. Friction is a force, which is related to acceleration, not direction of motion. Conclusion 3 is correct, but you have the wrong reason: the reason is Newton's third law. – David K May 31 '17 at 18:09
• @DavidK The 1,2 conclusions were obtained from physics.stackexchange.com/a/94837/128083 – mathnoob123 May 31 '17 at 18:11
• @DavidK Please also review the edits at 1,2. – mathnoob123 May 31 '17 at 18:14