SINCE 1828
law of motion
Definition of law of motion
1
: a statement in dynamics: a body at rest remains at rest and a body in motion remains in uniform motion in a straight line unless acted upon by an external force
— called also Newton's first law of motion
2
: a statement in dynamics: the acceleration of a body is directly proportional to the applied force and is in the direction of the straight line in which the force acts
— called also Newton's second law of motion
3
: a statement in dynamics: for every force there is an equal and opposite force or reaction
— called also Newton's third law of motion
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Merriam-Webster unabridged
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