section_path: Physics/Mechanics/Conservation of motion
section_name: Conservation of motion
paragraph_text: Conservation of motion
  
  In Principles of Philosophy (Principia Philosophiae) from 1644 Descartes outlined his views on the universe. In it he describes his three laws of motion. (Newton's own laws of motion would later be modeled on Descartes' exposition.) Descartes defined "quantity of motion" (Latin: quantitas motus) as the product of size and speed, and claimed that the total quantity of motion in the universe is conserved.
  
  If x is twice the size of y, and is moving half as fast, then there's the same amount of motion in each.
  
  created matter, along with its motion... merely by letting things run their course, he preserves the same amount of motion... as he put there in the beginning.
  
  Descartes had discovered an early form of the law of conservation of momentum. He envisioned quantity of motion as pertaining to motion in a straight line, as opposed to perfect circular motion, as Galileo had envisioned it. Descartes' discovery should not be seen as the modern law of conservation of momentum, since it had no concept of mass as distinct from weight or size, and since he believed that it is speed rather than velocity that is conserved.