 Potential energy of gravity. Potential energy, again, is energy which is stored. And in particular, we're looking at when it's stored such that a mechanical force is ready to do some work. Now, in the case of gravity, what I'm talking about here is that if a mass is lifted upwards, then the gravity force is ready to pull it back down. So that's going to do some work. If I take a look at that work, what I see is that the potential energy stored is equal to the outside mechanical work needed to prepare the system. And in this case, preparing the system means lifting the mass up. So for gravity, that external work needed to lift an ass up is going to be given by mg y. And that means the potential energy is going to be an mg y. So our equation for the energy stored by gravity is the potential energy of gravity is equal to mg y. Now, you'll also see it written very commonly as mg h. So we'll discuss that one, too. In both of these equations, m is the mass, and g is gravity. That acceleration of gravity are 9.8 meters per second squared. And the y, or the h, is the vertical position, or height. So it really means the same thing. It's just some textbooks tend to use h. Some textbooks tend to use y. Gravity, in terms of the energy, it could be expressed not just as pe sub g, but e of gravity. So that'd be the energy of gravity. Or some textbooks use a capital U for potential energy, so it'd be u sub g for the potential energy of gravity. And some textbooks don't even differentiate between what type of potential energy you have. So they'll just use a capital U for the potential energy of gravity, or any other potential energy. Let's look at a quick example here. If I've got a 50 kilogram object, and it's lifted 3 meters up in the air, what's my potential energy? Well, if I go back to my equation here, what I see is that my 50 kilograms gets plugged in for m, my 3 meters gets plugged in for y, and g is a value that's a constant, so we know that on Earth it's 9.8 meters per second squared. So plugging all three of those values in, what I'd get is 1,470 joules. Now a quick note on the units here, if I've got kilograms times meters per second squared times meters, well, that's going to give me kilogram meters squared per second squared. And that is equivalent to my unit of joule. So there's your potential energy of gravity.