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Law of conservation of momentum states that
For two or more bodies in an isolated system acting upon each other, their total momentum remains constant unless an external force is applied. Therefore, momentum can neither be created nor destroyed.

The law of conservation of momentum says that momentum only moves from one place to another, since it's neither created nor destroyed.

So let's say that the truck barreling down the highway crashes into a small car, causing it to slow down. It just lost momentum. But momentum can't be destroyed, so where does the momentum go? It goes into the small car, which will now be moving at a different speed (probably faster) than it was before. The momentum transferred from the truck to the smaller car. Momentum was conserved.

Consider the example of an air-filled balloon as described under the third law of motion. In this case, the balloon and the air inside it form a system. Before releasing the balloon, the system was at rest and hence the initial momentum of the system was zero. As soon as the balloon is set free, air escapes out of it possesses momentum. To conserve momentum, the balloon moves in a direction opposite to that of air rushing out.

b)

Given:

Mass of bullet = 10g

Velocity of bullet = 200ms^-1

Mass of pistol = 3 kg

Due to the law of conservation of momentum the momentum of the bullet is equal to the recoil of the gun. So the sum of the momentum of both the objects is equal to zero.

M_bV_b = –M_gV_g , Now ,

$V_g =-\frac{M_b{V}_b}{M_g}=-\frac{0.01\times 200}{3}=-0.66 m/s$

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