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Homework Help: Rotation question. Plz . Im so confused

  1. Nov 16, 2009 #1
    Rotation question. Plz plz plz help. Im so confused

    1. The problem statement, all variables and given/known data

    Two masses m1 and m2 are connected by light cables to the perimeters of two cylinders of radii r1 and r2 respectively, as shown in the diagram above. The cylinders are rigidly connected to each other but are free to rotate without friction on a common axle. The moment of inertia of the pair of cylinders is I = 45 kg.m2. Also r1 = 0.5 meter, r2 = 1.5 meter and m1 = 20kg.
    a) Determine m2 such that the system remain in equilibrium

    Then the mass m2 is removed and the system is released from rest.
    b) Determine the angular acceleration of the cylinders
    c) Determine the tension in the cable supporting m1
    d) Determine the linear speed of m1 at the time it has descended 1 meter.

    2. Relevant equations

    I = mr^2

    3. The attempt at a solution
    a) I tried 2 ways but I dont know which one is right
    sum torque of the system = 0
    r1 * tension 1 = r2 * tension2
    (0.5)(9.8)(20) = (1.5)(9.8)m2
    => m2 = 6.7

    or I = sum mass * r^2
    => m2 = 17.7

    and for the next parts of the question, I tried but I'm not sure they are right
    b) tangential acceleration = g = 9.8
    angular acceleration = a/r1 = 9.8/0.5 = 19.6
    c) T = mg = (9.8)(0.5) = 4.9
    d) I know we use the kinematic equation of [tex]\theta[/tex] = [tex]\omega[/tex]t + 0.5[tex]\alpha[/tex]t2 but since we dont know what t is, i guess we cant substitute it in
    any help is appreciated. thanks a lot. I have trimester exam coming up next week
  2. jcsd
  3. Nov 17, 2009 #2


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    Homework Helper

    Re: Rotation question. Plz plz plz help. Im so confused

    a) if the torques sum to zero the system is static equilibrium,... vertical forces are supported by the shaft,

    i'm not sure what your are attempting by summing the masses...

    for b) cut the problem into two FBDs:
    - one for the mass, write down the vertical force/acceleration balance (what is unknown?)
    - the other for the cylinder, write down a torque / angular accerlation balance

    connect the 2 systems through the tension in the string & c) & d) will follow
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