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Homework Help: Two blocks connected with spring and pulley

  1. Jun 7, 2016 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1= 20.0kg connected to a block of mass m2 =30.0 kg by a massless string that passes over a light, frictionless pulley. The 30.0-kg block is connected to a spring that has negligible mass and a force constant of k =250 N/m as shown in Figure P8.64. The spring is unstretched when the system is as shown and the incline is frictionless. The 20kg block is pulled a distance 20cm down the incline of angle theta = 40 degrees and released from rest find the speed of each block when the spring is again unstreched

    2. Relevant equations
    Ei =Ef

    3. The attempt at a solution
    Iam confused if we should have 0.5Kx2 as the energy in the system finally. This how I think the equation should be set up but Im not sure if my reasoning is right for the 1/2kx2 being there (the mass of the block will compress the string isnt.
    Taking the intial point from when m2 is 20cm down and m1 is 20cm up :

    M2gh + 1/2kx2 = 1/2(M1 + M2)vf2 + M1gh + 1/2kx2

    Would this be correct. At the final position the spring will compress 0.784 since:
    M2g = kx
    X = (20)(9.8)/250
    X = 0.784

    So is my reasoning correct or am I having misconceptions
  2. jcsd
  3. Jun 7, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Two problems I see:
    - The final position is with the spring unstretched. So what does that tell you about the spring PE at that point?
    - Realize that the masses do not have the same change in vertical position, since one is on an incline.
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