Fig. 3-1 Momentum gained by the rocket is equal in magnitude to the momentum of the expelled gas.

An object initially at rest may break into many parts in an explosion or disintegration. These pieces move in different directions with different speeds. Since the initial momentum of the object is zero, according to conservation of momentum, the sum of the final momentum of the pieces should also add up to zero. When a rocket launches, it expels a lot of hot gas vertically downwards. By conservation of momentum, the gain in upward momentum of the rocket is equal in magnitude to the downward momentum of the expelled gas (Fig. 3-1). The recoil of guns is another interesting example that serves to illustrate the principle of momentum conservation.