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Homework Help: Double Pulley Systems/Complex Pulley System Problems

  1. Feb 2, 2012 #1
    This is a question I received as part of my homework

    Your great grandmother gave you an antique scale that her great grandmother gave to her. It consists of a square platform mounted on 4 identical
    springs. When you step on the platform, you compress the springs, causing
    the platform to descend slightly, A needle indicates by how many centimeters you have descended. Unfortunately the conversion to useful units (like
    stone, or poundals) has been lost. However, your great-grandmother also
    gave you an extra spring, and on its label it says that the spring constant
    is 1.5 × 10
    4N/m. When you get on the scale you cause the platform to
    descend by 0.98cm. What is your mass?

    I have no idea how to go about solving this problem or what equations to use but, do I have to use the principles of conservation of energy for this? Equating gravitational to spring?

  2. jcsd
  3. Apr 24, 2012 #2
    Hi mk8993 ,if I'm not wrong then this is d solution:
    When you got on the platform you applies a force(=your weight) on it which causes all the 4 springs to descend.Suppose your weight is W.
    From spring const. eqn. F=kx,since all the 4 springs descend through the same distance then the force is distributed into four(number of springs) parts according to their spring constants i.e here, x being constant the more the value of k the more F will be.Here all springs have same value of k so,for one spring W/4=1.5*10^4*0.98*10^-2⇔W=588N
    so your mass is =588/9.8=60kg[assuming g=9.8m/s^2]
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