Rotational kinetic energy of a flywheel

AI Thread Summary
The discussion focuses on calculating the kinetic energy stored in a flywheel for a car, with specific parameters including a radius of 1.12 m and a mass of 736 kg, rotating at 7270 rev/min. The user initially converts the rotational speed to rad/s but encounters an error in their kinetic energy calculation, suggesting a possible misunderstanding of the moment of inertia formula. It is proposed that the flywheel should be modeled as a disk, using the equation I = 0.5MR^2 instead of I = MR^2. The user seeks assistance in correcting their calculations to determine how long the flywheel can power the car before needing to be recharged. Accurate modeling and calculations are essential for solving both parts of the problem.
mfm578
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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.
 
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Is it possible the flywheel is modeled as a disk rather than a hoop, in which case

I = .5MR^2, not I = MR^2 ?
 
Yes, try!

ehild
 

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