Newton's cannonball A cannon on top of a very high mountain shoots a cannonball horizontally. If the speed is low, the cannonball quickly falls back to Earth (A, B). At intermediate speeds, it will revolve around Earth along an elliptical orbit (C, D). Beyond the escape velocity, it will leave the Earth without returning (E). Newton's cannonball was a thought experiment Isaac Newton used to hypothesize that the force of gravity was universal, and it was the key force for planetary motion. It appeared in his posthumously published 1728 work De mundi systemate (also published in English as A Treatise of the System of the World). ## Theory In this experiment described near the start of his De mundi systemate, Newton visualizes a stone being projected from the top of a high mountain, and "that there is no air about the earth, or at least that it is endowed with little or no power of resisting". As a gravitational force acts on the projectile, it will follow a different path depending on its initial velocity. If the speed is low, it will simply fall back on Earth. If the speed is the orbital speed at that altitude, it will go on circling around the Earth along a fixed circular or