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Homework Help: Help and Fast please

  1. Dec 7, 2004 #1
    Help and Fast please!!!

    A car is designed to get its energy from a rotating flywheel whth a radius of 2 m and a mass of 500 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5000 rev/min. (a) Find the kinetic enrgy stored in the flywheel. (b) If the flywheel is to supply energy to the car as would a 10 hp motor, find the length of time the car could run before the flywheel would have to be brought back up to speed. 1 hp = 746 Watts

    Can somebody walk my through this process....i already know I have to convert 5000 rev/min to radians/sec but i don't even know how to do that!!! please i really need this!
  2. jcsd
  3. Dec 7, 2004 #2
    5000/60 = 83.3 revolutions per second
    83.3 * 2pi = 523 rad/sec

    Remember that 1 revolution is equivalent to 2pi radians.

    The kinetic energy stored in the flywheel would be...

    KE = \frac{1}{2}I\omega^2

    I is equivalent to the moment of inertia and in this case, if i am not mistaken should be...

    KE = \frac{1}{2}m\omega^2r^2

    Remember that...

    Power = \frac{dW}{dt}

    so you should now be able to figure out b.
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