Newton and the Apple

There is a popular story that Newton was sitting under an apple tree, an apple fell on his head, and he suddenly thought of the universal law of gravitation. As in all such legends, this is almost certainly not true in its details, but the story contains elements of what actually happened.

What Really Happened with the Apple?

Probably the more correct version of the story is that Newton, upon observing an apple fall from a tree, began to think along the following lines: The apple is accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground. Thus, by Newton's second law there must be a force that acts on the apple to cause this acceleration. Let's call this force "gravity," and the associated acceleration the "acceleration due to gravity." Then imagine the apple tree is twice as high. Again, we expect the apple to be accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree.

Sir Isaac's Most Excellent Idea

Now came Newton's truly brilliant insight: If the force of gravity reaches to the top of the highest tree, might it not reach even further? In particular, might it not reach all the way to the orbit of the Moon? Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force, because the acceleration due to gravity could change the velocity of the Moon in just such a way that it followed a curved path around the Earth.