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Updated on January 16, 2014

Newton's Theory of Gravity

Newton wrote in 1687 in his most significant of works, "Philosophiae Naturalis Principia Mathematica," that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. His brilliance allowed him to deduce this law from his own observation of the cosmos. Perhaps even more intuitively brilliant was his realization that gravity effects orbiting bodies in space in precisely the same way that gravity effects objects falling to earth. How could such seemingly disparate phenomena actually be manifestations of the same law of gravity? In other word why do some things remaining in orbit while other things fall to the Earth?

A Thought Experiment

In 1728 in A Treatise of the System of the World Newton explained this strange phenomena by way of a thought experiment;

Suppose you fire a cannon ball, the ball's forward velocity will keep it in the air for a certain period of time depending on the speed it is traveling forward in comparison to the force of gravity. but unless it reaches escape velocity ( around 10,000 m/s) it will fall to the ground at the same rate that everything does (accelerating toward Earth at a rate of 32 feet per second). But suppose the cannon ball was shot with such velocity that the rate at which it traveled allowed the curvature of the Earth kept it in flight, always traveling far enough and fast enough to avoid impact with the earth as the planet fell away.. If this forward velocity was fast enough so that gravity could not bring the object to earth before it had traveled to a new point at which the Earth had fallen away, then it would never hit the ground but rather eventually orbit the earth and return to the point at which it started. All the while traveling between 7,000-10,000 m/s.

The Implication

This is precisely what Newton realized was happening to the orbiting heavenly bodies. They, caught in the gravity of the body that they orbited, were constantly falling toward those bodies. But at the same time they were traveling at such a forward speed that instead of crashing into the body, they followed an elliptical path around the more massive object. This balance of two forces allowing an object of remain in perpetual free-fall without ever hitting the body toward which it is falling became known as centripetal force and allows objects to travel in a curved path around more massive objects.

And so in precisely the same way that objects in our atmosphere inevitably falling to Earth, so to are all the 2,271 man made satellites, the moon, and every other kind of space debris, man made and otherwise, caught in the earth's gravitation. Newton realized that his proverbial ( and perhaps fictitious) apple, as it fell to the Earth, was being acted upon by the same force that kept the heavenly bodies in orbit. Gravity.


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      mbuggieh 4 years ago

      Great title! Got my attention!

      Interesting ideas.