 So what can you do with a light board? Well just like any other board you can write on it in different colors and even rub out your mistakes when you make them. Let me use a simple physics equation, simple piece of physics to illustrate this and we'll see if we can weigh the Earth. So here's my schematic of the Earth and here's me standing on the surface of the Earth. And we can imagine that I have a mass M that the Earth has a mass big M subscript E and we need one other quantity. We need to know the distance between me standing on the surface of the Earth and the center of the Earth. We'll call that R subscript E, the radius of the Earth. Now the equation that we need may be familiar to some of you from high school physics or from university and that's Newton's law of gravitation. And it says that the force, the force between two objects, in this case the force that's pulling me down to the center of the Earth is given by G, the gravitational constant, multiplied by the mass of the Earth, multiplied by my mass, divided by, well, we actually don't need the subscript E. Here we'll just call it R because we've only got one quantity of R to worry about, multiplied by the radius squared. So let's make the mass of the Earth the subject of that equation because that's what we're trying to find. That's F multiplied by R squared divided by G times M. Now if you remember the force that I feel pulling me down towards the center of the Earth, this quantity here is nothing more than my weight. And I can rewrite that as my mass multiplied by the acceleration due to gravity. So if we put that into the expression what we find is that my mass cancels out. So it doesn't matter whether we're talking about me standing on the Earth or you or any other object resting on the surface. Let's now put some numbers in and we can actually calculate the mass of the Earth. So G, the acceleration due to gravity, we have the mass of the Earth, is 9.81 multiplied by the radius of the Earth, which is 6378.1 kilometers, so times 10 to the 3 meters, and that's all squared, divided by the gravitational constant, which is 6.67 times 10 to the minus 11, and the units of that are Newton's meters squared kilograms to the minus 2. Use our calculator to work out all of that, we get 5.97 times 10 to the 24 kilograms. And if you go and look it up in a textbook or on the internet, that is in fact the mass of the Earth.