How to do it?
1 Answer
(A) -- (r)
(B) -- (q)
(C) -- (p)
(D) -- (s)
Explanation:
The explanation below is a bit detailed and lengthy - but most of it can be easily worked out in one's head. Since friction, with its limiting values, is often confusing to students, I have decided to err on the side of verbosity.
The friction between the two blocks will try to make them fall with the same acceleration, if possible. The strategy we shall adopt is to assume that the two blocks fall with the same acceleration, - and check whether it is possible for the friction between the two blocks to supply the force that will be needed for this. If not, we start over.
Since there is no horizontal movement the normal force exerted by the wall on the left block, and the two blocks on each other are each equal to 10 N.
The weights of the two blocks are 10 N each.
Unless the limiting value for the force of this friction were to exceed
Case (A)
Since
For this to be achieved the force of friction needed between the two blocks is zero. Since zero is well withing the value of limiting friction (sic) , this is what will happen!
So, the accelerations are
Case (B)
The net vertical force on the left block is
In this case, there being no friction between the blocks, the case was simple - and we had no need to adopt the general strategy.
Case (C)
The force of friction exerted by the wall is
This would mean that the two blocks, if they were to move together, would accelerate down at the rate
For this to happen, the net downward force on the right hand block must be
Case (D)
The force of friction exerted by the wall is
This would mean that the two blocks, if they were to move together, would accelerate down at the rate
For this to happen, the net downward force on the right hand block must be
So, here, the blocks will slip with respect to each other, and so the friction between them will be the limiting value. Since obviously the block on the right will move down faster, the force of friction the left hand block exerts on it will be upwards. This means that the net force on the right hand block is
The left hand block will have a net downward force of
and so its acceleration will be