Law of  Conservation of linear momentum   

“The total momentum of an isolated system of interacting particles is conserved” is the statement of Law of conservation of linear momentum.

Example 1:

When a rocket is fired the fuel burns releasing exhaust gases downward, pushing the rocket upward with an equal and opposite momentum. So the rocket moves upward

Example 2:

When a shell explodes into pieces, each piece flies off in opposite directions with equal momentum.

Example 3:

When a bullet is fired from a gun, the gun gives a kick in the backward direction.

Explanation:

(i) According to “Law of conservation of linear momentum”, the total momentum before and after firing should be same and is equal to zero.

(ii) So, as the bullet gets forward momentum, the gun will get equal and backward momentum.

Hence, the gun gives a kick in the backward direction.

Example 4:  

A heavy rifle not recoils as strongly as a light rifle using the same cartridge.

Explanation:

(i) According to “Law of conservation of linear momentum”, Recoil of the rifle (V) = mu/M

where, M – mass of the gun;  m – mass of the bullet ;  u – velocity of the bullet

(ii) The recoil velocity of the rifle is inversely proportional to mass of the rifle.

So, heavy rifle not recoil as strongly as a light rifle using the same cartridges.

Example 5:

If a bomb at rest explodes into two pieces, the pieces must travel in opposite direction.

Explanation:

(i) Bomb is initially at rest.  So, initial momentum of the bomb = 0

(ii) According to “Law of conservation of linear momentum”,

Initial momentum of the bomb = Final momentum of the bomb

negative sign indicates that the pieces  must travel in opposite direction.