Main content

# Law of conservation of energy

## Video transcript

- [Instructor] Get excited because in this video we're going to talk about one of the most important laws in all of science. And that is the law of conservation of energy. You'd be amazed by how much of the universe we can infer based on the law of conservation of energy. And you'll be amazed by how many holes you can poke in science fiction plots based on the law of conservation of energy. Let's just start with the language that you might typically see. And then we'll try to understand it a little bit deeper. So it tells us that the total energy of an isolated system is constant. Energy is neither created nor destroyed. It could only be transformed from one form to another or transferred from one system to another; I pretty much underlined the whole thing because it's so important. Now to understand this, let's just think about the types of energy that we have studied. We have studied things like kinetic energy, which is the energy due to an object's motion. We have talked about potential energy, which you could view as energy due to an object's position. And that would be the case of mechanical potential energy. If you were to combine these two types of energy, together, they're known as mechanical energy. So let me put a little box around it. That is mechanical energy. And when you're first learning physics, these are the types of energies that we focus on. But there are other types of energy. There's thermal energy. You have nuclear energy. You have chemical energy. And so these aren't the only forms. So when we talk about the law of conservation of energy, things like kinetic energy could be transformed into chemical energy. But we're not gonna talk about those other types of energies in this video. So to start to appreciate this, let's first think about how mechanical energy can be conserved. So you could almost view this as a law of conversation of mechanical energy. But then we're gonna make things a little bit more complicated and see if we can trip ourselves up and see if we can somehow defy the law of conservation of energy. And be very skeptical of anyone who claims that they can defy the law of conservation of energy. So let's start with a system that contains all of the Earth in a ball. So let's just call this the Earth-ball system. And when you're dealing with the law of conservation of energy, it's important to specify your system. And we're going to assume that it's an isolated system, that it's not interacting much with other outside systems, the things like the sun or whatever else. And so we have, I've drawn the Earth here, in this kind of grass flat thing. And then let me draw my ball. And let's say my ball is held above the Earth, just like that. So while the ball is stationary, and we'll assume that there's no air here. So while the ball is stationary like that, then we have all potential energy, we could call it gravitational potential energy. So it's all, and the symbol for potential energy we tend to use is U. And we could say this is gravitational potential energy. And we could say there's no kinetic energy. No kinetic energy. If we thought about a broader system, if we talk about the solar system or something like that, then the Earth is orbiting around the sun, the sun is orbiting around the central of the galaxy. But that's why we're specifying, this is the Earth-ball system. But what would happen if I were to let go of the ball? And especially what is the energy profile of the ball right as it touches the ground right over here? And I'm assuming it's just going to hit the ground and just not bounce in any way, that would complicate things. Well, in that situation, all of a sudden you have no gravitational potential energy, but right as it touches it, not when it's stationary, right as it touches it, it's gonna have a lot of kinetic energy. So all kinetic energy. And so what we saw, what we see here, is that that potential energy all gets transferred into kinetic energy right as that ball touches the ground. Now I know what you're thinking. But what about right after that moment? If that ball, especially if doesn't bounce, if it just sits there, it looks like we have no energy anymore. It looks like energy has been destroyed. So my question to you is where did that energy go? Pause the video and try to think about it. So some of you might say, "Hey, once the ball "has just, it's at rest there, "well, maybe we found a case "where we have defied the law of conservation of energy." And, remember, I told you, be skeptical if (chuckles) anyone ever tells you that. Where the energy has gone, it's been dissipated. It has gone into heat. It would have been converted into thermal energy. So the ball and the ground would actually get that much warmer because that kinetic energy, right as it touches the ground would be turned into thermal energy. So one again, we have not defied the law of conservation of energy. Now another thing you might say is, "Well, okay, imagine a world that there is air." So let me draw a bunch of air particles right over here. And we know that if as a ball falls down it goes through the air, you could consider that air resistance. Some people would call that the friction due to air. Well, that would slow that ball down. So maybe it would not have as much kinetic energy when it gets down here. And so it seems like energy would be destroyed in that situation. And, once again, I would tell you, no, the energy has not been destroyed. As the ball falls down, it's going to heat up the ball and the air around it. And so, once again, that air resistance, that is a dissipative force. It's going to result in the generation of thermal energy. And if we wanted to write this in terms of equations, there's a couple of ways to write this. We could write, if we're just writing the law of conservation of mechanical energy, and we're not talking about dissipative forces, we could say that the initial kinetic energy plus the initial potential energy is going to be equal to, is gonna be equal to your final kinetic energy, your final kinetic energy, plus your final potential energy. Now another way to write this exact same thing is to say that the change in kinetic energy plus the change in potential energy is going to be equal to zero, assuming we don't have any dissipative forces, and assuming that we're not converting into some of these other forms of energy, like chemical energy, or thermal energy. But if you wanted to include dissipative forces, dissipative forces do something called non-conservative work. They do actually negative work, because the force of, say, friction, is always acting opposite in the direction of motion. So to factor that in, we could rewrite these equations. We could write that your initial kinetic energy, plus your initial potential energy, plus any work done by non-conservative forces, this would be like air resistance or friction. And this would be negative right over here if we're talking about, say, friction. That is going to be equal to your final kinetic energy, plus your final potential energy. Or this one right over here, we could write your change, change in kinetic energy, plus change in potential energy is going to be equal to the work done by dissipative forces. And, remember, if we're talking about friction, dissipative forces, this right over here is going to be negative. Another way that you could've thought about this is we could've put in thermal energy, we could say that our change in kinetic energy, plus our change in potential energy, plus our change in thermal energy is going to be equal to zero. Or you can include the work done by a dissipative force. And so, for example, if you saw a situation where your total change in mechanical energy right over here was negative, you're not defying the law of conservation of energy. It's not the energy was destroyed, it's that you had this negative work done by those dissipative forces. And where did that energy go? It gets converted to thermal energy. So let's do a couple of other examples just to appreciate this. So let's do a Earth pendulum system here. So here, that's the Earth. And then I have some type of tower. And let's say I have a pendulum here. I have a pendulum. And at the low point, the ball just goes right over there, then it goes back up to that point. And let's say the difference in height between this point right over here, and this point where it is right over there, it is equal to H. And let's say this is the highest point that the end of the pendulum will get to. So at this point we are maximum potential energy. And right as it's about to change direction, we have no kinetic energy, it's gonna be stationary for just a moment. But then the pendulum's going to swing back. And when it gets right over here, all of that potential energy is gonna be converted to kinetic energy, assuming we don't have any dissipative forces like friction, slash, air resistance. And then all that kinetic energy gets converted back into potential energy. Another example that we could look at that would complicate this a little bit more is to think about an Earth spring ball system. So that's the Earth. And let's say there's a spring right over here. And we have a ball that starts stationary. So up here it's all potential energy, gravitational potential energy. So all gravitational potential energy. Assuming we have no air resistance, we let go. Right before it touches the spring when we have maximum velocity, here it is going to be all kinetic energy, all kinetic energy, but then it's going to compress the string. And assuming we don't have any thermal energy generated, you actually will always, in the real world, have some thermal energy generated, but if the spring gets compressed and at some point the ball is over here, well, now, you have some of that energy that's been converted into spring potential energy. Or sometimes it's called elastic potential energy, because by virtue of compressing that spring, this thing's going to rebound and it could be converted back into kinetic energy and/or gravitational potential energy.