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A safe held by a string breaks and lands on a spring

  1. May 26, 2009 #1
    1. The problem statement, all variables and given/known data
    A 1209 kg safe is 1.54 m above a heavy-duty spring when the rope holding the safe breaks. Thr safe hits the spring and compress it 53 cm. What is he spring constant of the spring?


    2. Relevant equations
    K=mV^2/2
    U=mgh
    Ki+Ui=Kf+Uf


    3. The attempt at a solution

    I thought I understood momentuem and energy but not anymore. Here is what I did....

    Ui=mgh= 2*9.8*1.54
    Kf=mV^2/2

    Then solved for V and got 5.494 m/s

    Then used that velocity for the spring constant by using......

    U=k(delta x)^2/2
    K=mV^2/2

    then solved for k and got 129912 N/m...... but that is coming up wrong what am i doing wrong.
     
  2. jcsd
  3. May 26, 2009 #2
    any ideas?
     
  4. May 26, 2009 #3

    PhanthomJay

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    Well for one thing you are using m = 2 kg when it is given that m = 1209 kg. Then you neglect the gravitational potential energy in the 2nd set of equations. It is better to do this all in one step: Initial energy = Final energy, where v_initial = v_final = 0.
     
  5. May 26, 2009 #4

    tiny-tim

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    Hi talaroue! :smile:

    (try using the X2 and X2 tags just above the Reply box :wink:)
    No, the safe stops moving 53cm lower. :wink:
     
  6. May 26, 2009 #5
    JAY:

    so your saying that i should just use U=mgh and U=k(delta x)^2/2....solve for k and get....

    k=2mgh/delta x^2

    TINY TIM:

    so I didn't go about the problem wrong I just have to add the .53 m to the hieght?
     
    Last edited: May 26, 2009
  7. May 26, 2009 #6

    Astronuc

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    The safe starts 1.54 m (154 cm) above the spring and then travels another 0.53 m after contact. That must be considered with respect to gravitational potential energy.
     
  8. May 26, 2009 #7

    PhanthomJay

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    Yes, but 'h' is as noted by others, and don't forget to correct your value of 'm'.
     
  9. May 26, 2009 #8
    ahhh I see at least I had the right idea just need to be more careful. If you guys don't mind I am having problems with another problem as well.... it might be a same problem i am going to go back and look at it but I made a thread about it as well....

    Second problem I have issues with
     
  10. May 26, 2009 #9
    I wasn't using 2 as my m, that was just noting the second equation is U=k(delta x^2)/2 I didn't combined them when i posted that, i see what you are saying though.
     
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