Question

In the figure, the blocks are of equal mass. The pulley is fixed. In the position shown, A moves down with a speed u, and vB = the speed of B.

(a) B will never lose contact with the ground.

(b) The downward acceleration of A is equal in magnitude to the horizontal acceleration of B.

(c) vB = ucos θ

(d) vB = u/cos θ

Answer 1

Correct Answer is: (a, d)

Let the lengths of the sections of the string be BP = l1 and PA = l2 in the position shown. Let B move through a small horizontal distance x to B' and A move down through a distance y to A'. Length of the string

= BP + PA = B'P + PA'.

l1 + l2 = (l1 - xcos θ) + l2 + y

or y = xcos θ

or vA = vB cos θ = u.