[SOUND] So we started talking about energy, about saying it was the rearrangement of chemical or nuclear bonds into more stable states. Then we've even done some mathematics. To be able to figure out how much energy is made by those rearrangements. The thing is that we've been talking about energy in a very abstract manner, a number. Something in the equation, the letter Q. But what does that really mean? The key is that energy, this energy that's released, is in the motion of the products. It's in the motion of the things that are in their more stable state. If I take the methanol and I burn it with oxygen, it turns into water vapor and carbon dioxide. It turns out that the water vapor in carbon dioxide are moving. They're moving really, really fast. In fact, they're on fire, they're moving so fast. That's where the energy goes. It goes into the motion of the more stable product. Same thing in the nuclear reaction. If I took deuterium and tritium and I fuse it into helium, the helium is moving many many, a thousand or a million times faster than the deuterium and tritium was coming into the reaction. It's in the motion of the end product. And that's what makes it so practical. Let's take that shot glass burning. It's very reminiscent of what happens in an internal combustion engine. You take a liquid hydrocarbon fuel and you add oxygen to it and set it on fire. It turns into gases, carbon dioxide and water vapor and they're moving very, very quickly. Moving so quickly, they can move against a piston and therefore, produce motion for the vehicle. Now we do have a way, using physics, to figure out exactly how fast they're moving or how fast given that amount of energy, something could move. And that's called kinetic energy. The formula for kinetic energy is the energy equals one-half times the mass, times the speed squared. In common physics language, it's one-half mv squared, v for velocity. But the real key is it gives us a means to say if I can convert and that's a big if. If I can convert all of that energy into motion of some mass that has a certain value, I can tell you how fast it will go. An example might be a concrete block, ten kilograms, 22 pounds. How fast could I make it go by burning a whole shot glass of methanol, a mole of methanol? What we do the math? We would find out that 161.6 kilo calories, converts first to joules, out of 667,000 joules. Take a ten kilogram mass, take that large number, divide by ten, multiply by two, take the square root and you get 368 meters per second. That's 815 miles per hour. That's an awfully fast brick. Now the brick doesn't really go that fast because no engine, no manner to convert the motion of those products into the motion of this macroscopic object is 100% efficient. Yet, we can still talk about the energy transfer using that simple format. There are other energy expressions as well. One is potential energy. Potential energy represents storage against gravity. We're all stuck here on the Earth. And I mean literally, right? My feet are on the ground and if I lift them up, they want to fall back down. It's because the Earth is very, very massive. Not get into what exactly gravity is at the moment, but we can use a simple formula for potential energy. So that we can actually use this to calculate say, the energy in a hydroelectric plant. When you store water at a certain height and let it fall. A formula is that the energy equals the mass times a constant for gravity. It would be different on the moon or Mars than it is on Earth, times the height, mgh. My same example, if I took this concrete block, ten kilogram block and I was going to actually launch it straight up. We know that if I put all that chemical energy into it and it can do it at 100%, it would be going 368 meters per second. And now if I launch it, how high will it go? I'll take the energy number, 667,000 joules equals the mass in kilograms. The gravity is 9.8 meters per second squared on Earth, it's around ten. Divide that by 100, I get number of something like 6,700 meters. 6.7 kilometers, four miles. That's where the clouds are. One shot glass of alcohol, 100% conversion takes this cement block and throws it up to the clouds. Of course, it's going to fall back down. That's one of the cool things that you can see how kinetic energy could turn to potential energy. By working up against gravity at the cloud level, that break would stop and it's going to starts way back down again. Ignoring things like air friction, terminal velocity and so forth, it would come back down with that same 368 meters per second is what it would hit at at the end. Conversion of kinetic to potential energy. A pendulum describes that very well. In fact, you can take a really big pendulum and you take the professor's head and put them up against the wall. And he'd hold this big, giant wrecking ball in his hands. And it would drop in the pendulum and it would go faster and faster and it would reach the bottom of the floor. It would go up on the other side and then it would start coming back. And it would come back and it's going faster and faster and it's coming right up towards my head. It would stop right where I started. Because I converted kinetic to potential energy. There's no other energy gain. It's probably a little bit of energy loss. The fulcrum, wherever that rope was connected, right? It has a little bit of friction and also the ball moving through the air has some friction. So as long is no one along the way gave that ball another swat, professor has no fear that his head will be crushed. Because the ball will come right to where he released it and continue going back and forth. A conversion of kinetic to potential energy. It was a fun kind of device called the Rube Goldberg device. I think probably because that was the first person who wrote little cartoons about it. That actually relies on this conversion of kinetic to potential energy. You have a ball go down a ramp, right? Potential to kinetic. It crashes into something, maybe releases a spring. There's potential energy in the spring. It then kicks out another ball. And this can continue onward and onward and onward. Dominos. All falling in a line. Again, they have a potential energy because they are all set up on edge. And after they move, that's converted to velocity where they move which of course, because their now end up a bit lower. [MUSIC]