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.
The alternation 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.
Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.
