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Particles connected with a string

  1. Feb 27, 2016 #1
    1. The problem statement, all variables and given/known data

    Two particle of mass m each are tied at the ends of a light string of length 2a. The whole system is kept on a frictionless horizontal surface with the string held tight so that each mass is at a distance ‘a’ from the center P (as shown in the figure). Now, the mid-point of the string is pulled vertically upwards with a small but constant force F. As a result, the particles move towards each other on the surface. The magnitude of acceleration, when the separation between them becomes 2x is

    1422623933462Capture.PNG


    2. Relevant equations


    3. The attempt at a solution
    http://2.bp.blogspot.com/_lRKubY9oOyI/RrGGAxqjRUI/AAAAAAAAAHQ/TtdYi47lpDc/s1600-h/Newton

    2Tsinθ = F
    T = F/2sinθ
    where sinθ = (a2 - x2)1/2 / a
    Net force on each particle is T.
    So T = maacceleration
    So, aacceleration = F/2msinθ

    However, the answer is F/2mtanθ. I don't get why. Please help?
     
  2. jcsd
  3. Feb 27, 2016 #2
    Draw a free body diagram of the mass- do you get the net force on it to be T? you are making some serious error-the motion is along the plane towards the other body so net force can not be T.
     
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