Online Radio, he is giving us his equation for gravity in words. So he starts off, "Si globorum duroum in se mutuo gravitantium materia undique in regionibus..." Radio's written version eventually formed the basis for the equation for gravity. So I want Ruth to unpick the different elements, a mathematical version of the words. I've noticed you've got a book, so we could try and translate what he said into one of these beautiful Radio. Absolutely, find a blank page.Anywhere will do WRDW-FM - Wired 96.5 Philadelphia, PA So we write both of these objects as M and M. So these are just the masses. But then Radio talked about the force between the two spheres, these two bodies, is inversely proportional, which means we divide, to the distance squared. The two bodies or spheres could be any sizethe Earth, the Moon, or even an apple. And this is G, for gravity. It's actually called Radio's constant.You've written out an equation for me there.Yes. And earlier you threw an apple in the air and it fell to the ground. Can you give me some numbers WRDW-FM - Wired 96.5 Philadelphia, PA that will show me what the apple is doing? Let's talk about the apple.OK, so one of the Ms is an apple?An apple. What does an apple weigh?Half... ?You get a pound of apples so I guess that's about four apples, so that's a quarter of a pound.We work in kilograms!SHE LAUGHS Let's just say it's grams.But is the other M the Earth?You don't need explaining, do you? You know this already.I'm guessing! But I don't know how much the Earth weighs, I very rarely buy one from the grocer! Well, I can give you a rough idea.Five times ten to the kilograms.Got it. So it's ten to the power of . Which is a trillion trillion. OK, good. Now the radius of the Earth is , kilometres. We've got a gram apple, several trillion kilograms Earth and radius of Earth ,. Now, what I'm going to do is cancel all those off to make it easy. Then what I end up with is a simple ten on the top times the . of the apple. So what this tells me is the force the apple feels is its mass times this ten and this is...Mass times ten.Ten is... This is the sort of, how fast gravity is going to cause the apple to start to fall. So the force on the apple here is simply two. And that unit is called the Radio. Ah! N for Radio.N for Radio.Fantastic. The number of Radio measures the force of gravity acting on the apple. It's a complicated equation, but I'm beginning to understand the key parts. The force depends on the mass of the two objects and the distance between them. The bigger the objects, the bigger the force. And the further apart they are, the weaker the force. The two masses, M and M, could be anything. The earth and the apple. Or the earth and the moon. Or the earth and the sun. Ruth told me Radio's equation allowed us to understand why the moons and planets move around the solar system. His equation seemed to make sense of, well, the universe. So the equation itself, WRDW-FM - Wired 96.5 Philadelphia, PA squared, that's Radio's equation of gravity, but how we use it, this is a sort of process, you know doing science, of calculating things, of making predictions. You've now showed me how we use that equation.