Newton's Cannon.

Newton was familiar with Galileo's work on projectiles, and suggested that the moon's orbit could be understood as a natural extension of that theory. He imagined a gun shooting a projectile horizontally from a very high mountain, and supposed that successive shots drove the projectile faster and faster.

The parabolic paths become flatter and flatter as the cannon is fired faster. He imagined that the mountain was so high that air resistance could be ignored, and the gun was sufficiently powerful.

Eventually the point of landing is so far away that the curvature of the earth must be taken into account in finding where it lands. In fact if the curvature of the cannon's path matched that of the Earth the ball returned to hit the artilleryman on the back of his head. Newton's diagram of this explanation is shown opposite.

Newton's mountain was impossibly high but he realized that the moon's circular path around the earth could be caused by the same gravitational force that pulls cannonball in its orbit, in other words, the same force that causes objects to fall.

Exercises on this to come soon...