Question

A given box gets crushed when a mass of M kg is kept on its top. If this box were to be kept in an elevator, with a mass m on its top (m < M), the box would get crushed if the elevator were to start

(1) ascending with an acceleration (M/m)g

(2) ascending with an acceleration (m/M)g

(3) ascending with an acceleration (M/m-1)g

(4) ascending with an acceleration (1-m/M)g

Answer 1

(3) ascending with an acceleration (M/m-1)g

For the box to get crushed, the apparent weight, of the mass m, should be come equal to the normal weight of mass M (= Mg). 

The apparent weight needs to increase. Hence the elevator must start ascending with an acceleration ‘a’, such that