1. The problem statement, all variables and given/known data A bolt of mass 0.3 kg falls from the ceiling of an elevator moving down with an uniform speed of 7 m/s. 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 ? The problem I am facing is that the answer in book is given as 8.82 J and it mentions that it will be the same even when lift is stationary. The answer that I get is 25.5 J as explained below. May be I am missing something here. 2. Relevant equations I am assuming that the observer is on ground and not in elevator i.e. frame of reference is ground. Also, g is assumed to be 10 m/s2. KE1 + PE1 = KE2 + PE2, where 1 refers to when bolt starts its fall and 2 refers to just before it hits the floor. Also for the bolt fall, v = u + at and v2 = u2 + 2 x a x s 3. The attempt at a solution The bolt falls from a height with initial velocity of 7 m/s since bolt will have same velocity as the elevator when it starts it's fall. When it hits the floor it has fallen a distance of (3 + 7t) since floor of elevator descends by 7t So, if the floor of the elevator when bolt hits the floor is taken as datum level for potential energy, then KE1 + PE1 = KE2 + PE2, where 1 refers to when bolt starts its fall and 2 refers to just before it hits the floor. .KE2 + PE2 = .5 x .3 x v2 + 0 Also, using kinematics, we have v = 7 + 10 x t and v2 = 72 + 2 x 10 x (3 + 7t) Solving above equations, t = .78 seconds, v= 14.8 m/s Since bolt does not rebound after hitting the floor, so KE2 + PE2 - (KE of bolt after hitting floor) = Heat generated. = .5 x .3 x 14.82 + 0 - .5 x .3 x 72 = 25.5 J. But answer at back of book is given as 8.82 J. I am not getting where I am making a mistake.