A bullet of mass 4.2 $$ \times $$ 10^$$-$$2 kg, moving at a speed of 300 ms^$$-$$1, gets stuck into a block with a mass 9 times that of the bullet. If the block is free to move without any kind of friction, the heat generated in the process will be

Work done for a certain spring when stretched through 1 mm is 10 joule. The amount of work that must be done on the spring to stretch it further by 1 mm is