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Homework Help: Rotational Dynamics

  1. Nov 13, 2004 #1
    Hello,
    I have a problem dealing with torques and acceleration. I am sure I solved it right but when submitting the answers, I get the response "Incorrect" :cry:

    The problem:

    A wheel of radius 2 m, mass 53 kg, and moment of inertia (3/4) (53 kg) (2m)^2 about the center of mass is mounted on a frictionless horizontal axle.
    (g = 9.8). A light cord wrapped around the wheel supports an object of mass 106 kg. The weight is released from rest at the level of A and falls a distance h, past level B (AB=28 m). a) Find the velocity as it passes B b) Determine the tension of the string.

    I set up everything.

    Torque = I* alpha = T*R since alpha = a/R and we know I, conclusion is T=42*a

    Then: T=m(g - a): putting T from above, I got a=7.0189189 m/s^2
    To find V as the mass passes point B: Vf^2 = 2*a*28m ---> and v at B = 19.83 m/s. This was OK

    For b) I used T=m(g-a), since I know a = 7.0189189m/s^2 , I substituted it at the equation and I got T= 294.79 N but this is not the right answer.

    Maybe I can use I*alpha = T*R and from that get T because everything is known, but I am not sure. Any suggestions?
     
  2. jcsd
  3. Nov 13, 2004 #2
    Can anyone help?
     
  4. Nov 13, 2004 #3
    For part a)
    F=mg
    Torque=mgr
    mgr=I(alpha)
    28=r(theta)
    Wf^2=Wi^2+2(alpha)(theta)
    Vf=r(Wf)

    For part b)
    a=R(alpha)
    T=m(g-a)

    Do these things help?
     
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