$A$ bolt of mass $0.3 \; kg$ falls from the ceiling of an elevator moving down with a 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?

(N/A) Mass of the bolt,$m = 0.3 \; kg$.
Height of the elevator,$h = 3 \; m$.
Acceleration due to gravity,$g = 9.8 \; m s^{-2}$.
Since the elevator is moving with a uniform speed,its acceleration is zero. The bolt falls relative to the elevator with an initial velocity of zero.
At the time of impact,the potential energy of the bolt relative to the floor is converted into heat energy.
Heat produced = Loss of potential energy = $mgh$.
Heat produced = $0.3 \times 9.8 \times 3 = 8.82 \; J$.
If the elevator were stationary,the relative velocity of the bolt with respect to the floor would still be zero at the start of the fall. Therefore,the heat produced would remain the same,$8.82 \; J$.