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Conservation of Energy?

  1. Mar 4, 2008 #1
    1. The problem statement, all variables and given/known data
    A 20.0 kg block is connected to a 30.0 kg block by a 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 240 N/m, as shown in the figure below. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0 kg block is pulled 18.0 cm down the incline (so that the 30.0 kg block is 38.0 cm above the floor) and released from rest. Find the speed of each block when the 30.0 kg block is 20.0 cm above the floor (that is, when the spring is unstretched).



    2. Relevant equations

    (K-U)sub(i)=(K-U)sub(f)

    3. The attempt at a solution

    i've talked with multiple physics teachers on this problem every body got different anwsers...

    i'm almost convinced the anwser is 1.1687 m/s^2

    Could you help me please?
     
  2. jcsd
  3. Mar 4, 2008 #2
    I need to see the figure. I can't tell the angle of the incline, wether there is one or 2 springs, and in what direction they pull etc.
     
  4. Mar 4, 2008 #3
    here is the image...

    A 20.0 kg block is connected to a 30.0 kg block by a 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 240 N/m, as shown in the figure below. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0 kg block is pulled 18.0 cm down the incline (so that the 30.0 kg block is 38.0 cm above the floor) and released from rest. Find the speed of each block when the 30.0 kg block is 20.0 cm above the floor (that is, when the spring is unstretched).

    [​IMG]
     
  5. Mar 4, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    You want conservation of energy--that's not quite it. Mechanical energy equals KE + PE. (You have multiple forms of PE to track in this problem.)
    Show how you got that.
     
  6. Mar 4, 2008 #5
    i know the parts just will you check my aritmatic?

    it's

    (K+U)initial=(K+U)final

    and i know all the spring constant parts with .5kx^2, mgh etc.
     
  7. Mar 4, 2008 #6
    please

    could you just check my arithmetic please?
     
  8. Mar 5, 2008 #7
    Show us your arithmetic so we can check it.
     
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