__Newton’s 4__^{th} Law

Now, what about the planets, Jupiter, and the tides? Newton explained that gravity reaches up from the surface of the earth and out into space, expanding and diminishing.

**The acceleration of gravity weakens in proportion to the distance squared.**

The gravitational field of a massive planet extends through space, but all the while weakens as the square of the distance from that planet. The distance is initially measured to be the radius of the planet, so the gravity would not be twice as weak when you are an extra meter above the surface, but a full earth’s radius.

This is usually written similar to the form of F=GMm/r^{2} and most people incorrectly believe that Newton didn’t know the value G, but that is another Isaac Newton myth.

Newton guessed that the density of the earth was between 5 and 6 times that of water. Which is correct. He then used the volume of a sphere to estimate the mass from mass = Density times "bulk", Newton's word for volume. Most people who teach physics are aware of Henry Cavendish’s experiment to determine the universal constant G. Except, that he was interested in measuring the density of the earth. The symbol G was introduced much later.

The one thing that Newton could not understand was how gravity acted through space. We now better understand that it is a gravity field that extends from the sun to the planets. This is the Einsteinian model, that mass bends the fabric of space-time, recently confirmed by the detection of gravitational waves that distort the gravitational field and travel at the speed of light.