If the resultant forces is zero, then from equation (7.3) we can write down

Equation (7.5) expresses the principle of conservation of linear momentum.

If two bodies collide with each other, then the force exerted by one body is+ F and the force exerted by the other is -F. The combination of the force i.e., F- F = 0. The net force acting on the system is zero, thus linear momentum will be conserved for this type of collisions.