# Universal Gravitation

Isaac Newton is the father of the Theory of Universal Gravitation.

What does the adjective “universal” mean?

You need to know that, before the publication of Principia Mathematica in 1687, scientists distinguished between two types of physical laws:

On one hand, the laws that governed the motion of terrestrial objects, (…) and on the other hand, the laws that governed the motions of celestial objects.

The best example of this, according to them, is the gravity that is responsible for the falling of bodies on Earth …

… but visibly non-existent in the heavens because the stars do not fall on our heads.

(…)

With a now famous illustration, Isaac Newton expounded the theory that the law of gravitational attraction is the same for the apple that falls from a tree

(…)

as for the Moon in its orbit around the Earth.

(…)

He imagined for this the following thought experiment.  A cannon on a mountaintop fires a cannonball.

The distance travelled by the ball depends on its initial speed.

Released without any forward speed, the ball would fall like the apple from the tree.

(…) Conversely, if the speed of ejection is high, then the cannonball can travel a great distance before hitting the ground.

Newton explained that the cannonball could even, under certain conditions, never hit the ground because of the sphericity of the Earth.

We do not say that it is no longer subject to gravitational force. On the contrary, it is permanently subjected to gravity, and is, as it were, in a state of perpetual free fall.

(…)

If not, it would follow the path of a straight line.

(…)

The cannonball has become a satellite  and its trajectory is called an orbit. An orbit is generally elliptical in shape.

(…) but one can observe particular cases of an orbit that is almost circular, as is the case for the Moon.

In this animation, as in Newton’s drawing, the forces of friction are ignored.

If we had to take into account friction with the atmosphere, the cannonball would be slowed down and end up falling to Earth.

(…)

In thus unifying the Laws of Nature on Earth and in the Heavens, Newton extended the work of Galileo and Kepler and well and truly made the Law of Gravitation universal.

Autre version de l’auteur original:

In thus unifying the Laws of Nature on Earth and in the Heavens, Newton well and truly made the Law of Gravitation universal.