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From the Astronomical Dictionary

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Isaac Newton

Isaac Newton (1662 - 1727) was the founder of the science of mechanics. Newton gave the first useful explanation of gravity, in the form of Newton's Law of Gravity. With this he explained how the Moon orbits the Earth. Newton also invented differential calculus and explained many optical phenomena such as the spectrum. The unit of force is named newton (symbol: N) after him.

The three "ordinary" Laws of Newton are:

  1. (The Law of Inertia) An object upon which no forces act moves at constant speed along a straight line.
  2. An object upon which a force acts is accelerated with an acceleration that is proportional to the applied force and inversely proportional to the mass of the object.
  3. (The Law of Action and Reaction) For each applied force there is an equal but opposite reaction force.