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Rotational inertia

January 4th, 2006 · No Comments

Inertia is the what keeps moving things moving and sitting things sitting. Have you noticed that if you’re pulling a heavy wagon or cart it’s hard to get it going at first but then it doesn’t take much effort to keep it moving? That’s inertia. Changing the movement of something takes energy. The heavier it is, the more inertia it has and the more energy it takes to change it’s movement.

It also takes energy to rotate something. When your wheels start to turn, that takes energy. But once they’ve started turning, they keep turning until friction energy slows them down. Most tracks have a slope and then bend to a flat portion in the middle. When your car rotates from the slope to the flat part, it takes energy.

With rotational inertia, heavy things are harder to rotate, but the placement of the weight matters, too. If you put weight on the end of a stick and twirl it, it takes more energy than if you put the weight in the middle and twirl it.

Try this experiment. Get a baseball bat or a broom handle. Go outside where it’s safe to swing a large stick around. First hold the bat by each end and twist it lengthwise like this:

Rotational inertia figure one

Then grab the bat with one hand in the middle and twirl it like a baton like this:

Rotational inertia figure two

Which way took more energy to start and stop moving? The second way was harder because it had more rotational inertia.


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