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Motion (LAWS OF three principles)

LAWS OF, three principles or axioms which were laid down by Sir Isaac Newton: (1) If a body be started in motion, and if no force act upon it, that body will continue in motion in the same direction, and with the same velocity. Of course this cannot be directly proved by observation, no one being able to start a body in a portion of the universe free from other bodies which will attract it, and if he could start it on in a vacant space, he could not watch its subsequent progress through infinite space and eternity. But the length of time during which tops will spin or finely mounted pendulums will go in the exhausted receiver of an air pump, can be accounted for in no other way than by supposing the truth of the first law of motion. (2) Change of motion is proportioned to the acting force, and takes place in the direction of the straight line in which the force acts. (3) To every action there is always an equal and contrary reaction; or, the mutual actions of any two bodies are always equal and oppositely directed in the same straight line.