Rocket works on the principle of the law of conservation of:

Rocket works on the principle of conservation of momentum. In a rocket, the fuel burns and produces gas at high temperatures. These gases are ejected out of the rocket from a nozzle at the back side of the rocket. The ejecting gas exerts a forward force on the rocket which helps in accelerating. The mass of gases escaping per second is very small and their momentum is very large due to their tremendous velocity of escape. An equal and opposite momentum is imparted to the rocket which despite its large mass builds up a high velocity.