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In his Principia, Newton reduced the basic principles of mechanics to three laws:
    Every body continues in its state of rest, or uniform motion in a straight line, unless compelled to change that state by forces impressed upon it. The change of motion of an object is proportional to the force impressed upon it, and is made in the direction of the straight line in which the force is impressed. 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.
These laws are known as Newton's first law of motion, Newton's second law of motion, and Newton's third law of motion, respectively. In this section, we shall examine each of these laws in detail, and then give some simple illustrations of their use.

Richard Fitzpatrick 2006-02-02