GCSE Science/Newton's second law
Newton's Second Law
A motion of a body is described by its momentum. Momentum is the product of a body's mass and its velocity. Momentum has both magnitude and direction. Newton showed that bodies only change their motion when a force is applied, Newtons first law. The change in the momentum is equal to the applied force.
Derivation of Newton's second law
Consider a snooker ball of mass m moving with a velocity v1. The ball hits the cushion and bounces off, it now has a velocity v2. The change in momentum is mv1 − mv2 = m(v1 − v2). The change in velocity is the acceleration, and so the change in momentum and hence force F is mass times acceleration, F = ma. Thus Action and Reaction are equal but opposite in direction.