Question

In the figure, the ball A is released from rest when the spring is at its natural (unstretched) length. For the block B of mass M to leave contact with the ground at some stage, the minimum mass of A must be

(a) 2M 

(b) M 

(c) M / 2

(d) a function of M and the force constant of the spring

Answer 1

Correct answer is: (c) M / 2

The spring will exert the maximum force when the ball is at its lowest position. If the ball has descended through a distance x to reach this position, mgx = 1/2 Kx², where k is the force constant of the spring. For the block B to leave contact, spring force = kx = Mg