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Homework Help: This is very hard i dont know how to start, please guide!

  1. Feb 6, 2010 #1
    1. The problem statement, all variables and given/known data

    (Take g = 10 m/s2 in this question.) The ends of a light inextensible string, of length 0.7 m, are attached to two fixed points A and B at the same horizontal level, where AQ = 0.4 m and QB = 0.1 m. A particle of mass 0.2 kg is attached to the string at the point P, where AP = PB = 0.3 m. The particle hangs in equilibrium, with the string passing over a small smooth peg fixed at the point Q in the horizontal line between A and B at a distance of 0.1 m from B, as shown below. Give three reasons why the tensions in the three parts, AP, PQ, and QB, of the string have the same value, and find this value. Also, determine the angle between QA and the force exerted on the string by the peg. Refer figure below:


    The peg is now removed and a particle of mass m kg is attached to the string at the midpoint M of PB. The system hangs in equilibrium with PM horizontal, and AP and MB inclined to the horizontal at angles β and γ respectively, where cos β = 7/9 and cos γ = 1/3, as shown in the figure below. Calculate m


    2. Relevant equations

    3. The attempt at a solution

    I'm stucked and I have no idea on how to start, can anyone please kindly guide me on this? I'll truly appreciate and thanks alot
  2. jcsd
  3. Feb 6, 2010 #2
    Use the fact that both the horizontal and the vertical components of all the forces on the various weights must cancel.
    Check if you have copied the problem correctly. In the third line AP = PQ = 0.3m must have been meant.
  4. Feb 6, 2010 #3
    the question is correct though. Sigh :(
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