Newton's Three Laws of Motion

1. Every body preserves in its state of rest, or of uniform in a right line, unless it is compelled to change that state by forces impressed theron.
2. The alteration of motion is ever proportional to the motive force impressed: and is made in the direction of the right line in which that force is impressed. (F=ma)
3. 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.

Notes: (Newton 1642-1727), right line == straight line.