Newton's laws and momentum
An object keeps its velocity unless a resultant force acts (first law). The resultant force equals the rate of change of momentum, , which is for constant mass (second law). Forces come in equal and opposite pairs on different objects (third law). Drag grows with speed, so falling objects reach a terminal velocity.
≈ 40 minConservation of momentum and collisions
In any interaction with no external resultant force, total momentum is conserved, in every direction. In an elastic collision kinetic energy is also conserved, and the relative speed of approach equals the relative speed of separation. In an inelastic collision some kinetic energy becomes heat, sound or deformation. In two dimensions, conserve momentum separately in perpendicular directions.
≈ 40 min