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Rotational Inertia - Race between a ring and a disc

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We're going to look at a couple more demonstrations of rotational inertia. Now to review, rotational inertia is the property of an object that deals with a resistance in a change in a state of rotational motion. It depends upon the mass of the object and how that object is distributed from the axis of rotation. For the first demonstration, let's look at this aluminum disk This has quite a bit of inertia because much of the mass is distributed far from the axis of rotation. We're going to put this in motion with a motor and let it spin while we're doing something else. And it will take some time to spin down because of its large inertia or rotational inertia. So I'll get this thing going, I'll let it spin up here for about 5 to 10 seconds. And I've turned the motor off now, so now it's just slowing down. Now it will slow down because there is friction in the bearings, but it's going to take a while to do that because of its large rotational inertia. While that's happening let's take a look at this demonstration right here. Here I got two disk with the same radii. One of them is an aluminum ring, and the other is a piece of wood. Now if these have the same mass they have the same inertia but they don't have the same rotational inertia because that mass is distributed in a different way for this the mass is distributed uniformly throughout, for the ring all the weight will be distributed far from where the axis of rotation will be, which will be right through the center. And so this will have a greater rotational inertia than this wooden disk. What we're going to have them do is race down this incline, and the one with the greatest rotational inertia will loose the race because they'll have a greater resistance to a change in state of rotational motion. Now right now the state of rotational motion of both objects is zero; they're not rotating. Once I remove the piece of wood, one of them is going to rotate more quickly let's see which one that is. So the wooden disk rotated more quickly an one the race down the incline that makes sense because we said that the metal ring had greater rotational inertia, so there's a greater resistance to its change in a state of motion and the rings going to have a smaller rotational acceleration as it goes down the plane. Let's take a look now at our disk. You can see that it's still turning, its slowed down its almost stopped by now. But, if we could make the friction in the bearings much much less, this could continue spinning for a long period of time. If we made this a very large metallic disk, we could store a large amount of rotational energy in that disk. And we could keep that energy stored for a long period of time such devices are called fly wheels and they are used exactly for that purpose, energy storage.


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