Conservation of Momentum

Momentum is one of the most powerful concepts in physics. While Newton's second law relates force to acceleration, momentum gives us a way to analyse interactions between objects — collisions, explosions, and other events — without needing to know the details of the forces involved. The principle of conservation of momentum is one of the most fundamental laws in all of physics.

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Momentum

Momentum is defined as the product of an object's mass and velocity:

p = mv

where p is momentum in kg m/s, m is mass in kg, and v is velocity in m/s.

Momentum is a vector quantity — it has direction as well as magnitude. The direction of momentum is the same as the direction of velocity.

Worked Example 1

A 1200 kg car travels at 15 m/s due east. What is its momentum?

p = mv = 1200 x 15 = 18 000 kg m/s due east

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The Principle of Conservation of Momentum

In a closed system (no external forces), the total momentum before an event equals the total momentum after the event.

Total momentum before = Total momentum after