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Can the spring constant k be negative ?

  1. Mar 25, 2012 #1
    Can the spring constant "k" be negative ?

    Hi,
    I found a difficulty when solving this problem from my textbook :

    In the arrangement of Figure 7-11 (which is basically a standard block attached to spring resting horizontally), we gradually pull the block from x = 0 to x = +3.0 cm, where it is stationary. Figure 7-36 gives the work that our force does on the block. We then pull the block out to x = +5.0 cm and release it from rest. How much work does the spring do on the block when the block moves from xi = +5.0 cm to (a) x = +4.0 cm, (b) x = –2.0 cm, and (c) x = –5.0 cm?

    So I found the spring constant k:
    W = -0.5kx2
    0.9 = -0.5k(0.03)2
    k = -2000 N/m ??? of course it's wrong (the correct answer is 2000 N/m) but i couldn't find my mistake.
    Please help me :)
     

    Attached Files:

  2. jcsd
  3. Mar 25, 2012 #2

    Doc Al

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    Staff: Mentor

    Re: Can the spring constant "k" be negative ?

    Why do you have a minus sign? The work done (by you) when stretching a spring is positive.
     
  4. Mar 25, 2012 #3
    Re: Can the spring constant "k" be negative ?

    We are talking about the spring's work, if it was our work, does human being have a spring constant ? :

    W=F.d.cosθ / θ=180°
    So it will be negative
     
  5. Mar 25, 2012 #4

    Doc Al

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    Staff: Mentor

    Re: Can the spring constant "k" be negative ?

    Sure, the spring's work will be negative. So what? The spring constant is still positive.

    What makes you think that that is a diagram of the spring's work? It looks like the positive work that you do in stretching the spring.
     
  6. Mar 25, 2012 #5
    Re: Can the spring constant "k" be negative ?

    ok right so how would you answer to the problem ?
     
  7. Mar 25, 2012 #6
    Re: Can the spring constant "k" be negative ?

    ok thank you i found my mistake, thank you very much
    Kf - Ki = Wa + Ws

    since it goes from rest to rest then Kf - Ki = 0j so Wa = -Ws ;)
     
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