Newton's Three Laws of Motion.
Newton's 1st Law
Every body continues at constant velocity, unless acted upon by an external force.
Newton's 2nd Law
The change of momentum of an object is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed. If the mass is constant, then
Newton's 3rd Law
To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.