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Homework Help: Hookes Law

  1. Oct 24, 2012 #1
    1. The problem statement, all variables and given/known data

    Three identical 6.00kg masses are hung by three identical springs, as shown in the figure. Each spring has a force constant of 8.50kN/m and was 11.0cm long before any masses were attached to it. How long is each spring when hanging as shown? (Hint: First isolate only the bottom mass. Then treat the bottom two masses as a system. Finally, treat all three masses as a system.)

    2. Relevant equations
    F=-ma
    F=-kx


    3. The attempt at a solution
    I started with the bottom block.
    -ma=-kx
    (6.00kgx9.8m/s^2)/-(8.50kN/m)=x
    x=6.92m? and then i added 0.11m
    and its still wrong
    I also took the F=ma and divided by 1000 to get into(0.0588) kN and then divided it by 8.50kN and then dded .11m and still wrong

    I need to find all three block eventually
     
    Last edited: Oct 25, 2012
  2. jcsd
  3. Oct 25, 2012 #2
    That can't be answered without the diagram.
     
  4. Oct 25, 2012 #3
    This is how it is from top to bottom
    Ceiling
    -
    spring
    -
    block
    -
    spring
    -
    block
    -
    spring
    -
    block
     
  5. Oct 25, 2012 #4
    What was your answer from doing the calculation this way, and how do you know it was wrong?
     
  6. Oct 25, 2012 #5
    For the bottom spring I got 0.1169m
    Middle Spring 0.3376m
    Top spring 0.35075m

    I put it in an online program
     
  7. Oct 27, 2012 #6
    Still need help
     
  8. Oct 27, 2012 #7

    Doc Al

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    Staff: Mentor

    Note that the spring constant is in kN/m, not N/m.
     
  9. Oct 27, 2012 #8
    Yes I know, check my 3 answers i posted, they are with converting the kN
     
  10. Oct 27, 2012 #9

    Doc Al

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    Staff: Mentor

    Looks OK.
    Show how you got these last two.
     
  11. Oct 27, 2012 #10
    I added the weights together
    Mg=kx
    ((6+6)9.8)/(8500N/m) + .22 m =middle spring
    Top spring
    ((6+6+6)9.8)/(8500N/m) + 0.33m
     
  12. Oct 27, 2012 #11

    Doc Al

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    Staff: Mentor

    Ah... But they all start with an unstretched length of 0.11m.
     
  13. Oct 27, 2012 #12
    Yes, i did add in the unstrecthed length?
     
  14. Oct 27, 2012 #13

    Doc Al

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    Staff: Mentor

    For the middle spring you added in twice the unstretched length; for the top spring you added three times the unstretched length.

    Each spring has a total length equal to its unstretched length (0.11m for each) plus the amount of stretch (which varies).
     
  15. Oct 27, 2012 #14
    Why would i not add in 2 lengths because the middle spring is also carrying the bottom spring
     
  16. Oct 27, 2012 #15

    Doc Al

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    Staff: Mentor

    How does that affect its unstretched length?
     
  17. Oct 27, 2012 #16
    Oh now i get it, i should just only add in 1 unstrecthed length because im only calculating for 1 spring
     
  18. Oct 27, 2012 #17

    Doc Al

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    Staff: Mentor

    Right.
     
  19. Oct 27, 2012 #18
    Thank you
     
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