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###### A-level mechanics question

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1 year ago

Could someone explain why theres no friction at A??? The mark scheme ignores friction completely in this question:

Reply 1

1 year ago

Reply 3

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)

Reply 5

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.

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.

Reply 7

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.

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

Reply 8

1 year ago

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.

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

Your friend is in error.

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.

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)

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