# Flywheel

**Classical Mechanics**Level 3

A bus sometimes has a flywheel. When the bus brakes, translational kinetic energy from the bus is transferred to this flywheel. When the bus starts up again, energy from the flywheel is then transferred back to the wheels of the bus.

Suppose a \(15\:000\:\text{kg}\) bus is initially traveling at a speed of \(20\:\text{m/s}\). During the braking process, 20% of the kinetic energy is transferred to a flywheel. This is a uniform solid disc that can rotate about its central axis; it has a radius of \(1.50\:\text{m}\) and a mass of \(300\:\text{kg}\). What will be the approximate rotational period of the flywheel?

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