Total momentum before collision=Total momentum after collision
Gases through the tail nozzle of the rocket move in the downward direction with tremendous speed or velocity. In order to balance the momentum of the gases, the rocket moves up. Though the mass of gases emitted is comparatively small, they have a very high velocity and hence a very large momentum. An equal momentum is imparted to the rocket in the opposite direction, so that, despite of its large mass, the rocket goes up with a high velocity.
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