[Music] Hi, it's Mr. Anderson and this is AP Physics Essentials video 79. It's on kinetic energy. And you might be thinking to yourself, why are we just talking about kinetic energy right now, so far into the course? Well, there's some subtleties of kinetic energy and potential energy that we really couldn't talk about until we get to this point. And so, remember, kinetic energy is energy of motion. If an object has mass and velocity, it's got kinetic energy. But let's say that object is simply rotating. Does it have kinetic energy right now? Sure does. It's rotational kinetic energy. Instead of using its velocity, we're using its angular velocity. But a good question might be, does it have potential energy at this point? And the right answer is no. In a classical sense, an object can have potential energy because an object is isolated from the system. We have to add another object. So if I add the earth to it, now it has potential energy. It's the earth is producing this gravitational field and there's potential energy stored in that baseball. And so if an object is in motion, then it has kinetic energy. It has to have mass and velocity. And remember the equation for kinetic energy is 1/2 mv^2. If we know the mass and the velocity, we can calculate its kinetic energy in jewels. Now that object could also be rotating. And if it's rotating like that, it still has rotational kinetic energy. And instead of using mass and velocity, we're using its moment of inertia or rotational inertia instead of mass. And then we're using its angular velocity. You can see the equation is essentially going to be the same thing. Now, kinetic energy is 1/2 I omega squared, where I is rotational inertia and omega is going to be the angular velocity. But remember, this doesn't have potential energy if it's in motion. We can only have potential energy. If there's another object or another part of this system, then it can have potential energy. Could be gravitational. It could be electric potential energy. Um, but remember, objects by themselves don't have potential energy. And let me use a little Ph simulation to get at that. So, if we take a pendulum and we lift it up on the Earth, it's going to have potential energy, which becomes kinetic, and then becomes potential energy again. It's going to oscillate back and forth. But watch what happens if I get rid of the Earth. And now there's no gravity and I simply let it go. Well, that object is going to stay there. It has no potential energy. There's no stored energy because it's a single object. Let me add the Earth again. Now, we've added that potential energy back. Let's say I pull the Earth away. What happens? Now, all that energy is just in kinetic energy or energy of motion. It's just going to keep moving like that. And so, this is gravitational potential energy. But remember, we could have electric potential energy as well. If we have a single charge like this, then there's going to be no potential energy. But if I add other objects, then we can convert some of that electric potential energy into kinetic energy. If we know the mass of the object and the velocity, we can figure out its kinetic energy. And the equation is pretty straightforward. If I were to pitch a baseball at around 90 m an hour, so a baseball weighs around 145 gram. 90 mph pitch is going to be 41 meters/s. So it's essentially add those values to my equation. And so it's 12.145 kg. Remember we have to convert grams into kilograms. And then I've got my meters/s. So I could figure that out to be 120 jewels of kinetic energy. So that energy is in the motion of the object. But it doesn't have to be moving to have kinetic energy. It could be simply rotating. And our equation is like this. It's 12 I omega^ 2. So let's say that's rotating like this. All I need to know is what is its rotational inertia and let's say that's 047 kg m uh squared. And then I have to know how fast it's rotating. So its angular velocity is 6.1 radians/s. And then I simply plug in those values. So I put those into my equation and I can figure out using significant digits that it's 87 jewels of energy just in that spinning baseball. And so as you pitch a baseball, it's going to have that translational kinetic energy, but also the rotational kinetic energy as well. And so did you learn to use a model to represent a single object that has kinetic energy? And can you calculate that? And then finally, do you understand that an object by itself can't have potential energy? We have to have another object or a system in order for us to store that energy due to position. I hope so. And I hope that was helpful. Hey. Hey. Hey. [Music]