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Anderson Video - Energy

Professor Anderson
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Hello class, Professor Anderson here. We're continuing our learning glass lecture on physics. I'd like to talk to you about an idea in physics that is very important, very prevalent. And this is the idea of energy. So, let me ask you guys first what you think about that word. So, John, I am going to pick on you first, what do you think about when you hear that word, energy? >> (student speaking) I think about potential energy -- I've taken physics before and -- >> Okay. >> (student speaking) Kinetic energy. But -- >> Potential energy, kinetic energy. >> (student speaking) How they can change from one to the next. >> Okay. So, when you start talking about words like this you might think, you know, what is it relate to my every day sort of life. And typically we use this word in regards to ourselves. I have a lot of energy today. Or I'm drained of energy. So, if I say that, if I say I have a lot of energy, what does that mean to you? >> (student speaking) That you are feeling upbeat, maybe. >> Okay. >> (student speaking) Maybe you're having a good day, feeling ambitious, good spirits. >> Okay. Ready to go. Ready to do things. Ready to move. Right? If I have a lot of energy I'm ready to go do things, I'm ready to get myself moving. And, so, this idea of movement is intimately entwined with this idea of energy. Where would I get the energy? If I say I've got a lot of energy today, where did I get that energy from? >> (student speaking) You probably had a big breakfast. >> Had food, right. Maybe I have coffee that has caffeine in it and it constricts my blood vessels and that changes my awareness of my energy level. Where's the food come from? >> (student speaking) Comes from plants and cornfields and variety of different things from the Earth. >> Okay. It has energy in it. If I eat it I can take advantage of that energy, right? It has chemical potential energy in it. And when I eat it I can release that energy and get myself moving. And you said it came from the fields, from the farms, right. Where did it get its energy from if it was, you know, a corncob growing in the fields. Where did it get its energy from? >> (student speaking) Well it had to grow, so, it had to get some sunlight, some water, probably planted in some good soil, that sort of thing. >> Okay. Sunlight, absolutely, had to get water from the ground, had to get nutrients from the ground, but it also had to get carbon from the air, right? Plants take in carbon dioxide and turn it into polymer structures, carbons structures within them, okay? And this is all energy captured. So, the idea with energy is just that. You're going to move energy from one location to another. It came from the sun, it went to the plants, the plants grew up, it went to me, I ate the plants, I have energy, I go run a race and now I've expended that energy, okay? When I expend that energy it goes somewhere else. It goes into heat. It goes into friction. It goes into thermal energy. And, so, energy is really interesting because energy can never be created or destroyed. Okay. Energy can never be created or destroyed. It's just there. It is there in our universe. There is some finite amount of energy in the universe and that's it. And all we do is move it from here to there. Okay? We are just transferring that energy. The sun is powered by nuclear fusion, right? That fusion creates a lot of heat and light. The light comes to the Earth, it gives energy to the plants, the plants grow up, we can eat the plants, we get energy and we just moving it all around the universe. So, it's sort of an interesting idea that you're never going to create energy, you're never going to destroy it, you're only going to move it around, okay? The energy in the universe is a constant number and it's finite. Okay, so, what can we say about the different forms of energy? Well, John mentioned one, which is kinetic energy. And kinetic energy is due to movement. If an object has a mass and it has a velocity, there's movement associated with it, that thing has kinetic energy. So, when I eat the food and I go run, I'm expending kinetic energy. One half MV squared is what we're going to learn. But there's also potential energy. And we call it potential energy, because it has the ability to make things move, such as gravity or springs. Okay, those are all forms of potential energy. If I compress a spring there is potential energy in it. Namely if I let it go it's going to spring back, it's going to send things moving. Gravity we know, if I take an object and I drop it, gravity has potential to get that thing moving. So, we say there is potential energy. There is also thermal energy. When stuff in the universe is bouncing around, atoms and molecules, that's what we call heat. And how fast they move is determined by the temperature. Okay? And so, any time atoms and molecules bounce around very quickly, that's more energy in it, more thermal energy, the higher the temperature. All right, let's take these now and put them into some mathematical terms. So, the first one we talked about was kinetic energy. We're going to write kinetic energy, with a capital K. And we said it's related to movement. And this is what it is defined as. And we're going to see in the next section how you can prove this, how you can derive it, but this is just a definition that we're going to give you at this point. It's one half the mass times the speed squared of the object. Okay? And now, one thing you notice about this equation right off the bat is it goes like the speed squared. So, if I double my speed, my kinetic energy goes up by a factor of four. If I triple it, it goes up by a factor of nine. All right? So, it is quadratic in the speed, but the other thing you notice is that this is a scalar quantity. I don't have any vector sign on the left, I don't have any vector sign on the right. Energy is a scalar quantity. All energy is a scalar quantity. Not just kinetic energy. Okay? It's a number. And the units are joules. So, let's take a look at this, right? We have kinetic energy. On the right we have units of kilogram and then V is meters per second, but we're going to square it. So, this is what a joule is. One joule is equal to one kilogram meter squared per second squared. Okay, that's the units of energy, the SI units of energy we're going to use in this class. Joules. Okay, so, that's kinetic energy. What about potential energy? Well, potential energy comes in a few different forms. One of them is gravity and gravity potential energy is MGH. The higher you go above the surface of the Earth the more potential energy you have. If I lift this pen higher and higher it has more potential energy, meaning, if I drop it it's going to be going faster when it comes back down the higher I start with. Okay? But we also have springs. We'll write those with a U sub of S. And this is one half KX squared. K is the spring constant. So, it is determined by the size of the spring. X is how far you compress it or stretch it from its equilibrium length. So, this one is also quadratic in this variable X. If I stretch it twice as far I put four times the amount of energy into its potential energy. Okay, those are some definitions. And we can now think about what happens in different situations and we're going to take advantage of these definitions.