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Homework Help: Unable to solve rotation KE problem using torque

  1. Aug 25, 2015 #1
    1. The problem statement, all variables and given/known data
    A war-wolf or trebuchet is a device used during the Middle
    Ages to throw rocks at castles and now sometimes
    used to fling large vegetables and pianos as a sport. Model it
    as a stiff rod of negligible mass, 3.00 m long, joining
    particles of mass m1 = 0.120 kg and m2= 60.0 kg at its
    ends. It can turn on a frictionless, horizontal axle perpendicular
    to the rod and 14.0 cm from the large-mass
    particle. The operator releases the trebuchet from rest
    in a horizontal orientation. Find the maximum
    speed that the small-mass object attains.

    2. Relevant equations

    3. The attempt at a solution
    I could get the answer using rotational KE concept but when do the same thing with torque, I cannot get the answer.

    Net torque = (60)(9.8)(0.14) - (0.12)(9.8)(2.86) = 78.96 Nm

    Find the moment of inertia of the system

    Center of mass of the system: [(0.12)(0) + (60)(3)]/ 60.12 = 2.994 m

    Moment of inertia at CM
    (0.12)(2.994)^2 + (60)(3 - 2.944)^2 = 1.078 kgm^2

    Moment of inertia = Moment of inertia at CM + MD^2 = 1.078 + (60 + 0.12)(2.994 - 2.86)^2 = 2.158 kgm^2

    Net torque = moment of inertia x angular acceleration
    78.96 = 2.158 x acceleration
    acceleration = 36.6 rads^-2

    s = 1.57 rad, a = 36.6, u = 0, v = ?
    v = 10.7 rad/s = 30.66 m/s, which is wrong

    Is that because the downward force by weight is not constant during the rotation, so that the torque and the angular acceleration I calculated will become wrong as they way I calculated it seems to treat everything to be constant during the rotation

  2. jcsd
  3. Aug 25, 2015 #2


    User Avatar
    Homework Helper

    No, the torque is not constant during the motion. How do you calculate the torque?
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