Newton's cannonball

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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.

[edit] The Experiment

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

If there was no force of gravitation (as was thought at the time) the cannonball should follow a straight line away from Earth.

So long as there is a gravitational force acting on the cannon ball, it will follow different paths depending on its initial velocity.

1. If the speed is low, it will simply fall back on Earth. 2. If the speed is very high, it will indeed leave Earth. 3. If the speed equals some threshold orbital velocity it will go on circling around the Earth along a fixed circular orbit just like the moon. 4. 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 rotating around Earth along an elliptical orbit.