Newton's law:
1. In the absence of a net external force, a body either is at rest or moves with constant velocity.(law of inertia)
2. The net external force on a body is equal to the mass of that body times its acceleration; F = ma. Alternatively, force is proportional to the time derivative of momentum.
3. Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction.
