Newton's Laws of Motion
Three sentences, half the universe
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Summary
In 1687 Isaac Newton summed up the mechanics of the universe in three laws: (1) An object keeps its state of motion unless a force acts on it. (2) Force equals mass times acceleration (F = m·a). (3) For every action there is an equal and opposite reaction. Rockets, cars, athletes: everything that moves obeys these three sentences.
In depth
Newton's Principia Mathematica (1687) is one of the most influential scientific works in history. His three laws explain all everyday-scale motion, from the orbits of the planets to a pencil falling to the floor (as long as the speed is not close to the speed of light and the object is not the size of an atom). First law (inertia): Newton's most revolutionary idea: force is needed not to keep an object moving, but to change its motion. An object launched in frictionless space keeps going forever. Second law (F = m·a): The quantitative link between force and acceleration, the equation that turned physics into mathematics. A Formula 1 car's acceleration depends on the force from its engine and how light the car is. Third law (action and reaction): A rifle recoils because the forward push on the bullet creates an equal and opposite reaction. Rockets work on exactly this principle: they push exhaust gas backward and are pushed forward in turn.
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