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Homework Help: Rotating cylinder - number of revolutions before it reach 2500 rpm

  1. Mar 25, 2007 #1
    1. The problem statement, all variables and given/known data

    A flywheel has shape as a homogeneous cylinder with mass m = 40,0 kg and radius r = 0,50 meters. The cylinder is rotating round the axis of symmetry, and is runned by a motor with constant angular velocity (w0). When the motor is switched off, the cylinder is affected by a moment of force that is caused by friction.

    When the motor is being swiched off, the rotational frequency of the cylinder was 5000 rpm. How many revolutions does the cylinder make before it stops?

    2. Relevant equations

    The friction: M = -kw
    w = omega
    k = (1,2 * 10^2) Nm/s
    w0 = 5000 rpm

    3. The attempt at a solution

    The moment of inertia: I = (m*r^2)/2 (cylinder)

    M = -kw => 1) w = -(M/k)

    w = 1/2*5000

    alpha = w/T = (1/2*w0)/T

    1) (1/2*w0) = -(M/k)

    (1/2*w0) = -((I*alpha)/k)

    Any help will be appreciated :)
  2. jcsd
  3. Mar 25, 2007 #2


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    I just solved this scenario and got

    [tex] \omega (t) = \omega_0 e^{\frac{-Kt}{I\omega_0}}[/tex]

    which means it's never going to stop. Because there is no element of static friction, the friction is tending to zero as omega.

    I must have made an error, maybe someone can correct me.
  4. Mar 25, 2007 #3


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    Homework Helper
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    Are you sure that's how the problem was worded? Some information seems to be missing, like how the flywheel was oriented, and where the friction acts (ie, axle, point in contact with the ground if vertical, or whole surface if horizontal).

    IMO, since the flywheel will be moving, there's always going to be dynamic friction till it stops.
    Last edited: Mar 25, 2007
  5. Mar 25, 2007 #4
    The following should help.


    http://islam.moved.in/tmp/3.jpg [Broken]
    Last edited by a moderator: May 2, 2017
  6. Mar 26, 2007 #5


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    Gold Member

    Thanks Nacer. I got the dimensions of K wrong and stuck the w0 in the exponential incorrectly.
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