With an example explain the basis of law of conservation of momentum.

When a gun is fired, the bullet leaves the barrel with some velocity, i.e. the bullet has some momentum. The bullet and the gun were at rest before the shot was fired. Therefore, initially the total momentum of both the bullet and gun was zero. When the gun is fired, the bullet acquires certain velocity and therefore some momentum as it leaves the gun. Since the momentum before and after firing should remain conserved, the gun must have momentum equal to that of the bullet, but in opposite direction. The gun therefore moves in a direction opposite to that of the bullet.