Math CentralQuandaries & Queries


Question from will, a student:

how much energy in KW do i need to start and subsequently rotate a 4 ton fly wheel of 1 metre radius at 12 rpm, please show how you have calculated this, thank you.

Hi Will.

Assuming the flywheel is a uniform solid disk, its rotational kinetic energy is ½Iω2, where I is the rotational inertia and ω is the rotational frequency. A solid cylinder has a rotational inertia of ½mr2, so the energy needed to spin it up is ½(½mr2)ω2 = ¼mr2ω2. This ignores friction or other influences of course.

Choose your units appropriately and you should be able to use this formula to calculate your answer, Will.

Stephen La Rocque.

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