Richard Muller, Professor of Physics, Berkley, California
Lecture 03: Gravity and Satellites
At 25:00 He says:
"What is the force of gravity on me? Well, its the pull of the Earth on my body. That is what we call the force of gravity. Newton figured out that the force of gravity is actually a force of attraction between mass. So, for example if you have the Earth, its a big mass here, and you have you, with your little mass here, that every atom of the Earth is pulling on every atom of you . . . you are also pulling on it. Ah you are pulling it up. You may say you don't have much mass, so you don't pull very much, but there are a lot of atoms here, and you are pulling on all of them. The amazing thing about gravity is that it goes RIGHT THROUGH THINGS; more effectively than even neutrinos. You are pulling, RIGHT NOW, on the atoms of the other side of the Earth. They're far away, so its not very strong, in fact your force of pull depends on the distance. The rule is given here. The force of pull is 1/r2, so we say the force of pull for gravity is a bunch of constants, G, mass of the first object, mass of the second object, divided by r2. . . . There is gravity between my two hands right now, the masses of first hand and second hand are kind of small, [not a lot of connections] when you plug in the constant here and so the force isn't very big. G is 6 * 10 to -7. But if you have enough fists and a whole Earth made of fists, then it all adds up. So we're being pulled in every which direction by ??? (he says gravity) and because the Earth is a sphere it feels as if, I mean the sideways forces cancel, that thing over there and over there are pulling me in opposite directions but they cancel each other. But the net effect is AS IF I am being pulled straight down.
Let me show you that again. Here is the Earth, here am I. Every bit of mass is pulling me in its own direction. These forces are weaker than these cause this is closer, its 1/r2, go twice as far away the force is 1/4, 100 times away 1/100the squared times, so most of the force is coming from nearby stuff and most of it is pulling me straight down. Now suppose the Earth is not a completely uniform sphere like this, and suppose there is a hole over here. What do we fill that hole with? Well we could fill it with vacuum (conceptually) but those don't really EXIST. Lets fill it with something light. Lets fill it with oil. Now look at the gravity on me. The rock over here is pulling me down this way, but the oil over here is pulling me, but not very much. They no longer cancel. So if I have a very accurate meter I can search for oil using the gravity anomaly."