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PhysicsGCSE Higher & A Level

What is momentum?

Momentum is the product of an object's mass and velocity, and it is conserved in any collision or explosion with no external force.

Momentum explained

Because momentum is a vector, direction matters: assign a positive direction and give opposing velocities a negative sign before applying conservation.
Impulse, the product of force and time, equals the change in momentum, which is why crumple zones and airbags reduce the force by extending the collision time.

Key formula

p=mv,Ft=Δ(mv)p = mv, \qquad Ft = \Delta(mv)

Worked example

A 2 kg2\,\text{kg} trolley at 3 m s−13\,\text{m s}^{-1} hits a stationary 1 kg1\,\text{kg} trolley and they stick: combined velocity =63=2 m s−1= \frac{6}{3} = 2\,\text{m s}^{-1}.

Examiner tip

Write the conservation equation with signs before substituting numbers — examiners award the setup mark separately from the answer.

Momentum: common questions

Is momentum conserved in every collision?
Yes, provided no external resultant force acts. Kinetic energy, however, is only conserved in elastic collisions.
What is impulse?
Impulse is force multiplied by the time it acts, and it equals the change in momentum it produces.

Related terms

Newton's second law

Newton's second law states that the resultant force on an object equals its mass multiplied by its acceleration.

Kinetic energy

Kinetic energy is the energy an object has because of its motion, equal to half its mass times its velocity squared.

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