Fire a cannonball fast enough, and it never comes back down
Newton imagined a cannon on an impossibly tall mountain, firing horizontally. A slow ball falls to the ground nearby; a faster one lands farther away; fired fast enough, the ball's fall curves at exactly the same rate the Earth curves away beneath it — so it never lands at all. What speed does the ball need, and what does that reveal about gravity?
Reveal the answer
At roughly 7.9 km per second — Earth's orbital velocity at sea level, ignoring air resistance — the cannonball would circle the planet forever, endlessly falling but never landing: an orbit. Newton used the thought experiment in 'A Treatise of the System of the World' to show that the same force pulling an apple down also holds the Moon in orbit — gravity and orbital motion are one and the same.
— Isaac Newton, A Treatise of the System of the World — written c. 1685, published 1728