A bolt of mass 0.3 kg falls from the ceiling of an elevator moving down with an uniform speed of 7 m s^–1. It hits the floor of the elevator (length of the elevator = 3 m) and does not rebound. What is the heat produced by the impact? Would your answer be different if the elevator were stationary?

Given,
Mass of the bolt, m = 0.3 kg 
Speed of the elevator = 7 m/s 
Height, h = 3 m 
Since the relative velocity of the bolt with respect to the lift is zero, at the time of impact, potential energy gets converted into heat energy. 
Heat produced = Loss of potential energy  
                      = mgh 
                      = 0.3 × 9.8 × 3 
                      = 8.82 J 
The heat produced will remain the same even if the lift is stationary. This is because of the fact that the relative velocity of the bolt with respect to the lift will remain zero.