Moment of inertia, also called rotational inertia, is a measure of an object's resistance to rotation. It is analogous to inertia, an object's resistance to change in linear velocity. The rotational inertia of an object depends on the mass of the object, the shape of the object, and how the mass is distributed throughout the object's shape.
The most common example, which would be easier for you to demonstrate in class, is swinging a baseball bat from the handle end and the barrel end. A baseball bat is much easier to swing (rotates easier) when swung from the barrel end. This is because more of the mass of the bat is located closer to the axis of rotation -- your hands. The farther away the mass of an object is from the axis of rotation, the harder it is to swing the object, and the greater the rotational inertia.
The following illustration shows how rotational inertia is important in figure skating. When a figure skater changes position, he or she is redistributing his or her mass. Thus, every position has it's own unique rotational inertia. The rotational inertia of the skater in the figure increases from left to right as the skater lifts his arms and leg, redistributing more of his mass further from his axis of rotation.
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© April, 1998, Montana State University-Bozeman
