Question

A bullet hits a block kept at rest on a smooth horizontal surface and gets embedded into it. Which of the following does not change ?
(a) linear momentum of the block
(b) kinetic energy of the block
(c) gravitational potential energy of the block
(d) temperature of the block.

Answer 1

(c) gravitational potential energy of the block

Explanation: 

In this case velocity of the block will change, hence (a) and (b) will change. Some part of the K.E. of the bullet will be converted into heat energy due to friction between bullet and the block during the embedment. Hence (d) will also change. Since height of the block does not change hence (c) will not change.