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The Law of Gravity & Mass

You've spent your whole life on the surface of a fairly large planet, so gravity is one of the most important forces in your everyday life. As intuitive as gravity may seem, however, the underlying physics is a little complicated. Newton's law of universal gravitation is the simplest way to understand gravity and holds true for most circumstances. For a more-complete understanding of how gravity actually works, however, you need Einstein's general theory of relativity.

Newton's Law
- The story is often told that a bonk on the head from a falling apple gave Newton the inspiration he needed for his famous law of gravity. It's not clear, unfortunately, whether this story is actually true. Regardless of how he thought of it, though, Newton was the first to realize the same force that causes an apple to fall also acts between Earth and the sun, the sun and the moon and indeed all other objects with mass. He also found he could calculate the strength of this force with a fairly simple equation.
Implications
- The basic formula for Newton's law of gravity is as follows: force of gravity = (gravitational constant) x (mass of object 1) x (mass of object 2) / (distance between the two objects)^2. This force always acts along an imaginary line connecting the center of mass of the two objects. Newton's law worked very well for explaining countless observations that had remained unexplained up until that time, like the orbits of the planets.
Challenges
- Despite his success, Newton found the nature of the gravitational force a little puzzling. He thought that action at a distance was "inconceivable." Nor did he care for the idea that gravity was somehow inherent to matter. Consequently, the way in which this force worked remained somewhat mysterious. By the late 19th century, however, scientists had begun to find some observations they couldn't reconcile with Newton's description of gravity. In particular, they found that Mercury's orbit precessed or changed the orientation of its axis very slightly over time.
Relativity
- General relativity completely changed the way scientists think about gravity. Rather than describing it as a force exerted through action at a distance, relativity described it in terms of space-time curvature. Basically, mass distorts space-time in the same way a heavy object would distort a stretched plastic tarp if placed in the middle of it. Objects follow a curved path when they travel through this curved space-time, and this effect is what causes a rock to fall or a planet to remain in orbit around the sun. Relativity makes much more accurate predictions, but in most ordinary circumstances Newton's law is a good approximation, so you can still use Newton's law to calculate how matter will behave under the influence of gravity.

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