3.6.2—Momentum and impacts

For a system of colliding particles, total momentum before impact equals total momentum after impact when the external impulse during the collision is negligible: Σmu=Σmv.

Keep signed velocities, identify the system, and use restitution only as a second equation. For an explosion or separation, the same momentum principle applies even though kinetic energy may increase.

A 2 kg trolley at 3 m s⁻¹ collides with a stationary 1 kg trolley and they move together: their common speed is (2×3)/3=2 m s⁻¹.

Momentum is a vector and can cancel; kinetic energy is not generally conserved in an inelastic impact.