Class 11 Physics Laws of Motion Conservation of Momentum

Conservation of Momentum

  • In an isolated system, the total momentum is conserved.

Example 1. In a Spinning top, total momentum = 0. For every point, there is another point on the opposite side that cancels its momentum.


Example 2. Bullet fired from a Rifle

Initially, momentum = 0

Later, the trigger is pulled, bullet gains momentum in àdirection, but this is cancelled by rifle’s ß momentum. Therefore, total momentum = 0


During the process, the chemical energy in gunpowder gets converted into heat, sound and chemical energy.

Example3.  Rocket propulsion                                                           

Initially, mass of rocket: M. It just started moving with velocity v

Initial momentum = Mv


Later, gases are ejected continuously in opposite direction with a velocity relative to rocket in downward direction giving a forward push to the rocket.

Mass of the rocket becomes (M-m)

Velocity of the rocket becomes (v + v’)

          Final momentum = (M –m) (v + v’)

Thus, Mass * velocity = constant

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