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Homework Help: Is my tutor wasting my time?

  1. Mar 21, 2008 #1
    1. The problem statement, all variables and given/known data

    This is the first part of a more complex problem but I shall give all the relevant data. A cast iron flywheel is rotating at 80rev/sec and is brought to rest in 20sec by a hydraulic braking system acting on the outer dia. of the flywheel. There is 4Nm of frictional torque in the flywheel system and co-efficient of friction of the brake interface is 0.4. I need to find the force that the brake is exerting on the flywheel. For info the brake is then connected to 2 hydraulic cylinders, of different areas, in series and then to a second order lever. However, if I can find the force at the brake I can crack the rest. I have not been given a moment of inertia or a radius for the flywheel and the more time I spend trying to solve this, the more I think that my tutor has made a mistake in not giving one of these.

    2. Relevant equations

    initial ang. velocity is 502.65 rads/sec before the braking

    T=Fr and F=(co-eff. of friction)*N

    Therefore T=co-eff.*N*r

    3. The attempt at a solution

    I started trying to use the above equations but the only T value I have is for frictional torque.
    The only moment of inertia value I can get is also using this fric. torque which is incorrect, I think.
    By converting rev/sec to rads/sec=502.65 I can also find the radians rotated by the flywheel. radians=1/2 (initial velocity+final velocity)*t
    =5026.5 radians
    Still cant seem to get anywhere with this. Do I need a moment of inertia or radius to get any further?
  2. jcsd
  3. Mar 21, 2008 #2
    There's indeed no way you can do it without the moment of inertia. I
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