When the total force acting on a system is zero, the linear momentum of the system remains constant for all time. This fact is called the principle of conservation of linear momentum.
An application is seen in the recoil of a gun when a shell is fired. Before the firing, the linear momentum of the shell and gun is zero, so the total linear momentum after the firing is also zero. Therefore the mass of the gun must be very much larger than the mass of the shell to ensure that the gun's speed of recoil is very much smaller than the speed of the shell. The principle may also be used, for example, in investigating the collision of two billiard balls.
Subjects: Sports and Exercise Medicine — Mathematics.