The Garden of Learning

Science

Which scientist proposed the three laws of motion?

  • Isaac Newton
  • Galileo
  • Kepler

The answer is Isaac Newton. In 1687.

Newton, in the Principia of 1687. The first law - that a body keeps moving unless something acts on it - is the counterintuitive one, because everything in ordinary experience stops. It required recognising friction as a force rather than as the natural tendency of things to come to rest, which Aristotle had assumed for two thousand years.

The third is the one most often misunderstood. Every action has an equal and opposite reaction does not mean forces cancel out and nothing moves; it means the two forces act on different objects. When you push a wall the wall pushes you, and a rocket works not by pushing against the air behind it but by throwing mass one way and being pushed the other - which is why it works in vacuum.

The laws are also not universally true, which is a strange thing to say about the most successful physics ever written. They break down near the speed of light, where relativity takes over, and at atomic scale, where quantum mechanics does. What Newton wrote is an approximation - an extraordinarily good one, accurate enough to fly spacecraft to other planets, and wrong in ways that only show up at extremes he had no way of reaching.