Balance Wheels
This exhibit demonstrates the relationship between distribution of mass and angular momentum. Guests can distribute the individual masses attached to the wheels and roll the wheels down an incline. What happens to the wheel's motions as you move the masses around?Wheels with weights on the rim have high rotational inertia and are harder to start spinning than wheels without weight on them. When you visit, notice how slowly the weighted wheel starts to roll. Wheels with their weight near the center have less rotational inertia and will roll more easily.
This is seen when ice skaters spin. When their arms are extended their bodies spin slowly, but as the arms are drawn in close to their bodies, the angular velocity (spin rate) increases greatly.
The weights are moving in two directions at once: in a straight line down the ramp and in circles as the wheel spins. Each type of movement has inertia or resistance to the movement. The closer the weights are to the center of the wheel, the smaller the rotational inertia. These weights move in very small circles and have a small distance to travel. When the weights are on the edge of the wheel, they have to move in large circles and have more rotational inertia.
Every object has mass which is a measure of translational inertia, an object's resistance to change in its translational, or linear velocity. Each object also has a moment of inertia which is a measure of rotational or spinning inertia. Rotational inertia is the resistance to changes in its angular spinning velocity.
When the mass of the object is centered close to its center of mass, that object will rotate easier than if the mass is distributed farther from its center. Mass that is farther from the center of rotation travels a larger distance about that center and therefore must have a higher resistance to change than if the mass were close to the center.