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Homework Help: Rotational motion of flywheel

  1. Oct 26, 2009 #1
    1. The problem statement, all variables and given/known data

    A car is designed to get its energy from a rotating flywheel with a radius of 1.85 m and a mass of 678 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 3610 rev/min. Find the kinetic energy stored in the flywheel. Answer in units of J.

    If the flywheel is to supply energy to the car as would a 7.9hp motor, how long could the car run before the flywheel would have to be brought back up to speed? Answer in units of h.

    2. Relevant equations

    rotational KE= 1/2 I [tex]\omega[/tex]2
    [tex]\omega[/tex]=[tex]\omega[/tex]o + [tex]\alpha[/tex]t

    3. The attempt at a solution

    I found part 1 which was 82905777.99J but am stuck on how to approach part 2. I think I should use a rotational equation but I don't know angular position nor angular acceleration. Since they give me 7.9hp, I think I should convert it to work (1hp=746W) but don't know what to do with it. Any direction to take would be helpful.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 27, 2009 #2


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    Homework Helper

    Hi ba726,

    This is not converting to work; it is changing the units: that is, changing a power of 7.9 horsepower to the same power in units of watts.

    Once you have the power in watts, what is the relationship between power and time?
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