 Let's have a look at another interesting thing when we look at conservation of angular momentum. That is, an angular momentum depends yes on the angular velocity, but also on the rotational inertia. Now in the linear case, the linear inertia is the mass and there is basically nothing that we can do about it with mechanics. Well, we can go to nuclear equations, but if we don't have any nuclear reactions, we can't do anything about the mass. However, for the angular inertia or the rotational inertia, we can do something about it. Remember, it depends on the location of the mass in regards to the axis of rotation. Now here, I'm sitting on a little chair that has a very good bearing. So initially, I can increase my angular velocity, my final velocity by adding some torque with my feet. But the moment that I let go with my feet, I should be going at more or less the constant angular velocity. Of course, there's a little bit of friction, so I'm going to slow down the beginning. Now, what do you think happens if I start rotating with a high rotational inertia? How do I create that, a high rotational inertia? I put my mass as far away from the axis of rotation, which is here as possible, so I spread them out. And then I tuck them in, and I don't touch the floor anymore, so there's no more torque. So if I tuck my hands in, my rotational inertia goes down. So according to this equation, what do you think happens to my angular velocity? Let's have a look. Okay, I'm accelerating with torque. Now I'm letting go, so no more torque, and now I'm putting my hands in and out again. So the effect is probably not so big, because I'm not spinning that crazy, but figure skaters, you can watch how this works. So let me do the effect a bit bigger. I'm going to take masses, a lot of mass, and I put them out. I'm going to spin a little bit. I'm putting them closer to myself. I'm going fast, and I'm going slow. And I can go fast again. Let me spin again. Spin, spin, spin, spin. Now I'm going fast. In the opposite, if I go out, I should slow down. If I go in, I should get faster again. Let me do this in the other direction to not get too dizzy. So big rotational inertia, low angular velocity. Now I'm putting my inertia small, which gives me a bigger angular velocity, and then at the end I stop myself by applying some torque from the ground. So unlike conservation of linear momentum, conservation of angular momentum really has a couple of very unexpected things happening, and always think about it as well as it is a vector rock.