# A-level mechanics question

Could someone explain why theres no friction at A??? The mark scheme ignores friction completely in this question:
If for part a) they take moments about A, then the forces acting at A have 0 moment. So the plank weight, weight of P and tension would be the 3 forces with non-zero moments as the desired expression seems to want.
(edited 1 year ago)
No thats not what I mean. I mean just the diagram. Why is there no friction at A? For part c, although you cant see it, you have to take moments arond B. I just want to know why the mark scheme, model answer, videos etc. dont draw a friction force downards at A?
Original post by MelonCrazy
No thats not what I mean. I mean just the diagram. Why is there no friction at A? For part c, although you cant see it, you have to take moments arond B. I just want to know why the mark scheme, model answer, videos etc. dont draw a friction force downards at A?

Can you upload the full question and the mark scheme youre referring to?
The wall is rough so there will be an upwards friction force on the plank at A. Note the diagram has no forces at all marked on it.
(edited 1 year ago)
Original post by mqb2766
Can you upload the full question and the mark scheme youre referring to?
The wall is rough so there will be an upwards friction force on the plank at A. Note the diagram has no forces at all marked on it.
v (diagram) or Y (model solution) is the frictional force on the plank at A.
(edited 1 year ago)
My friend explained it. Friction is actually acting on the rope, at point C, rather than at A. At A there is no friction since the rod is "attached" at point A, so like a nail in the wall (which of course wont have friction). Obviously, we're not considering forces acting on the rope, thats why friction is ignored.

V and H are NOT frictional forces, they are the horizontal and vertical components of the reaction force at A.
Original post by MelonCrazy
My friend explained it. Friction is actually acting on the rope, at point C, rather than at A. At A there is no friction since the rod is "attached" at point A, so like a nail in the wall (which of course wont have friction). Obviously, we're not considering forces acting on the rope, thats why friction is ignored.

V and H are NOT frictional forces, they are the horizontal and vertical components of the reaction force at A.

Question states that "... rod rests with its end A against...". It is not attached at A, and V (as per your diagram in post #6) is a frictional force.

There is no friction involved with the rope; it is attached at C (and B).
Original post by MelonCrazy
My friend explained it. Friction is actually acting on the rope, at point C, rather than at A. At A there is no friction since the rod is "attached" at point A, so like a nail in the wall (which of course wont have friction). Obviously, we're not considering forces acting on the rope, thats why friction is ignored.

V and H are NOT frictional forces, they are the horizontal and vertical components of the reaction force at A.

Agree with ghostwalker and I agree that H is not a frictoinal force, it is indeed the wall reaction, which is perpendicular to the surface. However v is the frictional force of the rough wall acting upwards at point A to the plank. The plank is not attached to the wall.

Also as youre considering the equilibrium of the plank, whatever occurs at C plays no part in the calculation. For the plank to be in equiilibrium, the vertical and horizontal forces acting "on the plank" (and their moments) need to sum to zero.
(edited 1 year ago)