Conservation of Momentum
Consider an example-when there happens a collision between moving train & bus , then nothing happens to train and bus gets totally crushed , why?
Also, when train halts at the station, then it takes some time to do that.
Thus, by seeing above examples we can say that heavier bodies (like train) takes some time to bring it's velocity to zero when brakes are applied though train and bus are moving with the same velocity. Therefore,to describe the motion of body , not only velocity is considered but mass is also taken into account.
The total quantity of motion possessed by a moving body is known as the momentum of the body. It has both magnitude and direction and hence a vector quantity.it is denoted by p.
magnitude of p=mv
Where m is mass of body
v is velocity of body
In the absence of external forces , the total momentum of the body is conserved.
Example-when the bullet is fired , it moves in the forward direction and the gun kicks backward. It is because before firing total momentum of system constituted of barrel and gun is zero. When bullet is fired it gains some momentum(due to velocity acquired by it) but to nullify this momentum gain, gun moves in backward direction such that it has momentum equal in magnitude to momentum of bullet with opposite direction.