Momentum

Momentum of an object is the product of its mass m and the velocity v. Momentum being the product of  a scalar (mass) and a vector (velocity), it is a vector quantity. The direction of momentum is that of the velocity. 

The momentum of a system remains constant in the absence of an external force. This is called the principle of conservation of (linear) momentum, and is a very powerful law to solve physical problems.  

Consider an object of mass m moving with velocity v collides with an object of mass M moving with velocity V. Assume that the objects do not  lose their masses during collision but their velocities after collision become  v' and V' respectively. If the force in the system is only due to collision and there is no external force, then, the principle of  conservation of momentum can be written as : 

                                                             mv + MV = mv' + MV'

Test Yourself

(1) An object of mass 2 kg moving on a smooth surface with velocity of 20m/s towards east collides with a stationary object of mass 1kg on its path. If they coalesce and move together, predict their common velocity after collision.