Physics

You have a grindstone with a mass of 90.0 kg, 0.340-m radius, and is turning at 90.0 rpm. You
press a steel ax against it, bringing it to a stop in 36.0 s. (a) What is the change in kinetic energy of
the grindstone? (b) If the tangential force due to the friction between the ax and the grindstone is
4.00 N, how many rotations will the grindstone make as it comes to a stop?

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  1. calculate I of solid disc

    omegai = 90 rev/min * 2 pi rad/rev * 1 min/60 s
    = 9.42 radians/second

    Ke = (1/2) I omegai^2 at start, zero at finish

    Torque = 4 R = 4*.34 newton meters

    alpha = Torque/I

    omega = omegai - alpha t
    omega = 0 at finish
    so
    t = omegai/alpha
    which is time to stop
    then
    d = total radians turned to stop
    d = omegai t - .5 alpha t^2
    divide that by 2 pi radians/revolution to get number of revolutions

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    posted by Damon

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