Posted by **Chan** on Monday, November 23, 2009 at 3:07am.

hi,need help please.thank.

A car is designed to get its energy from a rotating flywheel (solid disk) with a radius of 2.00 m and a mass of 525 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 4000 rev/min.

(a) Find the kinetic energy stored in the flywheel.

(b) If the flywheel is to supply energy to the car as would a 20.0-hp motor, find the length of time the car could run before the flywheel would have to be brought back up to speed.

a) Use the formula E = (1/2) I w^2, where w is the angular speed in rad/s and I is the moment of inertia.

ok this what I try:

first I convert 4000rev/min to rad/s

4000rev/min(2pi/rad/rev)(1.0min/60.0s)

=418.9rad/s

than I plug in E=(1/2)(525kg)(418.9rad/s)=109961.25J the #'s are too big.

(b) Time = (Full speed energy)/Power

You will have to convert horsepower to Watts when using the formula.

This part I'm not sure how to convert in Watts. Sorry but, I am really confuse.Thank again for your time.

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