3.1.3—Linear momentum as the product of mass and velocity

Momentum p=mv is a vector in the direction of velocity, measured in kg m s⁻¹. It is conserved for an isolated system when external impulse is negligible.

Use signed velocities on one axis and define the system before writing conservation. Momentum is distinct from kinetic energy, which depends on speed squared.

A 2 kg object moving at −3 m s⁻¹ has momentum −6 kg m s⁻¹ in the chosen positive direction.

Equal and opposite momenta can sum to zero while individual objects still move.