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Homework Help: Rotational motion -angular energy+angular acceleration

  1. Oct 19, 2005 #1
    The question is,

    You have a system of a weightless rod (length=1.00m)
    with three masses suspended rigidly as indicated, with m1 and
    m3 at the ends and m2 at 0.7m.

    (m1=20kg, m2=30kg, m3=20kg)

    The whole system is suspended from its centre-of-mass and is rotated with 25J of rotational kinetic energy. How many revolutions per second does it make in its rotational motion?

    (The picture provided shows a rod attached to the ceiling by a rope. and the three masses are attached to the rod at the distances mentioned in the problem above (kinda like a seesaw))

    I have a feeling that this question is worded wrongly..b/c I thought the only way that we can get a period T for this type of probelm is when the angular velocity is constant, but obviously in this problem there is angular acceleration (b/c there is a net torque). Therefore the angular velocity is actually changing constantly, so how can the rotational kinetic energy and the period be constant?

    I hope I have explained it clearly. If not, please ask me for further clarification. Any help from you is much appreciated!
  2. jcsd
  3. Oct 19, 2005 #2


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    I think the idea behind the question is that the rod is rotating with a kinetic energy of 25 J, and they want to know what angular speed this gives you. If the energy is constant, and assuming no losses due to friction, then there would be no acceleration.
  4. Oct 19, 2005 #3
    Thanks for the reply.

    However, what really bothers me is that there is apperently a net torque about the centre of mass, so does that mean there should be angular acceleration?

    Or am I thinking too much? :shy:
  5. Oct 20, 2005 #4


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    I thought at first you were assuming there was a net torque because of the differing masses, but the problem states that the rod is hung from its COM. Why do you think there's a torque?
  6. Oct 20, 2005 #5

    I got it now. So the whole system includes the rod and the masses......

    I thought it is suspended at the centre of mass of the rod.....

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