# Newton's cannonball

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Newton's cannonball

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 1728 book A Treatise of the System of the World.

## The experiment

In this experiment Newton visualizes a cannon on top of a very high mountain.

If there were no forces of gravitation or air resistance, then the cannonball should follow a straight line away from Earth.

If a gravitational force acts on the cannon ball, it will follow a different path depending on its initial velocity.

1. If the speed is low, it will simply fall back on Earth. (A and B)
2. If the speed is the orbital velocity at that altitude it will go on circling around the Earth along a fixed circular orbit just like the moon. (C)
3. If the speed is higher than the orbital velocity, but not high enough to leave Earth altogether (lower than the escape velocity) it will continue revolving around Earth along an elliptical orbit. (D)
4. If the speed is very high, it will indeed leave Earth. (E)

## Other appearances

• An image of the page from the System of the World showing Newton's diagram of this experiment was included on the Voyager Golden Record  (image #111).

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