Guys, so we're going to be talking a lot about energies in the next couple of videos and works. In this video, I want to briefly introduce you to the different types of energies that we'll be discussing, and also specifically kinetic energy. Let's check this out. So, what is energy? You'll be looking through your textbooks and talking with your professors, and you'll notice that there isn't a well-established definition for energy. The best I can come up with is that energy is just a physical quantity or property that some objects have. Now, scientists have debated this for years. We don't really know what exactly energy is, but fortunately for us, we do know how it works, and that's more important. So, let's talk about that. The unit for energy that we'll use is called the joule. It's represented by the symbol or letter J. The basic idea is that energy exists in many different forms throughout the universe. We've got things like thermal energy, heat. We've also got light energy, sound energy, electrical energy, and kinetic and potential energies, among others. But the main point here is that energy in the universe can't be created or destroyed. The way that energy gets transferred and shuffled around the universe is basically just getting transferred between these forms. Let me show you some examples that you're probably familiar with. You plug a light bulb into a power outlet, and you've basically converted electrical energy that's stored inside of the electrons in that electrical energy into heat and light. There's also energy that's associated with motion. For example, you push a toy box against a spring. You basically store some energy inside the spring when you're pushing up against it, and then when it gets released, that energy has to get transferred to the speed of the toy. So, basically, what's happening here is that you are transferring potential energy that's inside the spring—specifically, it's called elastic potential, which we'll talk about soon—and this is becoming kinetic energy. I want to talk about kinetic energy here, which you'll notice is associated with an object's motion. Kinetic energy, which we'll write as the symbol KE, is really just the energy that is due to an object's motion, specifically an object's speed. The idea here, I'm just going to show you the equation for kinetic energy:
KE = 1 2 m v 2 depends on your mass and your speed. Notice I haven't put any arrows on this formula. Energies, when we're talking about these energies here, are scalars. They are not vectors. Specifically, kinetic energy is always going to be positive, and it's never going to have a direction associated with it. If you're moving to the right and to the left with the same speed, you have the same energy. It doesn't matter the direction. Let's go ahead and take a look at this problem here. We want to calculate the kinetic energy of a 5-kilogram box moving to the right with 3 meters per second in part a. This box has a mass of 5, and it's moving to the right with a velocity, sorry, a speed of 3. If we want to calculate the kinetic energy, we just plug this in 1 2 m v 2 . So, this is just 1 2 5 × 3 2 . If you go ahead and work this out, what you're going to get is, I believe, this is 22.5 joules. Now let's look at part b. Part b, now you're going to have the same 5-kilogram box, except it's moving to the left now with 2 meters per second. So, this box, and now it has a speed to the left, and the speed here is 2. Remember we said the kinetic energy is a scalar. It only depends on your mass and speed. So we don't actually care about the direction of this speed here. So this kinetic energy, 1 2 m v 2 , this is 1 2 5 × 2 2 . If we go ahead and work this out, what you're going to get is 10 joules. So you have 10 joules, regardless of whether you're moving to the left or right, it's always going to be a positive number that we see. Alright? So that's it for this one, guys. Let me know if you have any questions.
