Rotational kinetic energy of a flywheel

[mfm578]

Homework Statement

A car is designed to get its energy from a rotating flywheel with a radius of 1.12 m and
a mass of 736 kg. Before a trip, the flywheel is attached to an electric motor, which
brings the flywheel’s rotational speed up to 7270 rev/min.
Find the kinetic energy stored in the fly-wheel.
Answer in units of J.

Homework Equations

KE = 0.5*Inertia*w^2
Inertia = m*r^2

The Attempt at a Solution

convert 7270 rev/min to rad/s = 761.314 rad/s

KE equation: 0.5*(761.314)^2*(736 kg)*(1.12m)^2 = 267554029.7

answer is apparently wrong... help please?




PART 2, I don't think I can do this without a correct answer from part 1

If the flywheel is to supply energy to the car as would a 11.8 hp motor, how long could the
car run before the flywheel would have to be brought back up to speed?
Answer in units of h.

 

[Kabbotta]

Is it possible the flywheel is modeled as a disk rather than a hoop, in which case

I = .5MR^2, not I = MR^2 ?

 

[ehild]

Yes, try!

ehild