||This article relies largely or entirely upon a single source. (December 2008)|
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, in the direction that it was fired.
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) for example horizontal speed of 0 to 7000 m/s for Earth
2 If the speed is the orbital speed at that altitude it will go on circling around the Earth along a fixed circular orbit just like the moon. (C) for example horizontal speed of at approximately 7300 m/s for Earth
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) for example horizontal speed of 7300 to approximately 10000 m/s for Earth
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).