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Gravitation is one of the four fundamental interactions of nature, along
with electromagnetism, and the nuclear strong force and weak force.
Modern physics describes gravitation using the general theory of
relativity by Einstein, in which it is a consequence of the curvature of

spacetime governing the motion of inertial objects. Newton's law of
universal gravitation provides an accurate approximation for most
physical situations.

There is a famous 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 and causes 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.
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 around 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 way that it followed an orbit

around the Earth.

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