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# KE/Rotational Motion

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Hi. Can someone please show me how to do the following problem? I posted it earlier, but I still can't get the correct answer using the OTA's very brief explanation.

Here's the question:

"A car is designed to get its energy from a rotating flywheel with a radius of 2.20 m and a mass of 499 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5120 revolutions per minute. Find the kinetic energy stored in the flywheel."

Here is the OTA's explanation:

"KE = 1/2 M V^2

M = moment of inertia = mass * radius^2 * inertial constant

mass = weight/g
assume inertial constant = 1

V = rotational velocity"

I think the problem is with the rotational velocity. I'm not sure how to arrive there. Is it not 5120 rev/minute = 5120 rev/60 s = 85.2 rev/s?

Solution. Denote the moment of inertia, the radius and the mass by , R and M, respectively. Then . Now we need to convert Rev/Min to m/sec first. Since the radius is R, we know that the rotational speed . So,
, ie., .

We know that . So the kinetic energy stored in the flywheel is

##### Solution Summary

"A car is designed to get its energy from a rotating flywheel with a radius of 2.20 m and a mass of 499 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5120 revolutions per minute. Find the kinetic energy stored in the flywheel."

##### Solution Preview

Us the basic formula Kinetic Energy = ½ M V^2

V is the linear velocity, which we have to find out from ...

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