When a system disintegrates into two, three or any number of particles as a consequence of the result of internal forces, the linear momentum of the system must be conserved.

This condition, in every case, makes one of the fragments have a velocity and direction completely determined by  the conservation of linear momentum.

Let us look at a couple of examples:

	When a radioactive atom disintegrates, the nucleus must move in the opposite direction to the emitted particle, so that linear momentum is conserved.
	When a rocket takes off, the mass and velocity of the gases that escape from the rocket in the opposite direction determine the speed that the rocket can achieve due to the conservation of linear momentum.