Newton's Laws of Motion

1. Every particle moves in a stright line at constant speed unless compelled to change this motion by forces acting on it.
2. When a force acts on a particle, the motion of the particle is changed in the direction in which the force acts, and the acceleration of the particle is proportional to the force.
3. To every action there is always an equal and opposite reaction.

The original source of this document is: http://liftoff.msfc.nasa.gov/sb/academy/rocket_sci/orbmech/newton.html