Question

A boy puts a heavy box of mass M on his head and jumps from the top of a multi-storied building to the ground. What is the force exerted by the box on the boy's head during his free fall? Does the force greatly increase during the period he balances himself after striking the ground?

Answer 1

During free fall:
Acceleration of the boy = Acceleration of mass M = g 
Acceleration of mass M w.r.t. boy, a = 0
So, the force exerted by the box on the boy's head = M × a = 0
Yes, the force greatly increases during the period he balances himself after striking the ground because of the weight of the box.