# Newton's laws of motion

## Newton's laws of motion

Relations between the forces acting on a body and the motion of the body, formulated by Isaac Newton. The laws describe only the motion of a body as a whole and are valid only for motions relative to a reference frame. Usually, the reference frame is the Earth. The first law, also called the law of inertia, states that if a body is at rest or moving at constant speed in a straight line, it will continue to do so unless it is acted upon by a force. The second law states that the force F acting on a body is equal to the mass m of the body times its acceleration a, or F = ma. The third law, also called the action-reaction law, states that the actions of two bodies on each other are always equal in magnitude and opposite in direction.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on Newton's laws of motion, visit Britannica.com.

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