4.3.1—Momentum

Total momentum p=mv is a vector. If external impulse is negligible, Σm u=Σm v before and after a collision or separation.

Choose a positive direction, keep signs on velocities, and define the system before writing the conservation equation.

A 2 kg cart at 4 m s⁻¹ and a 1 kg cart at −1 m s⁻¹ have total momentum 7 kg m s⁻¹ before impact.

Momentum conservation does not imply kinetic-energy conservation; inelastic impacts can dissipate energy.