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Work Done by a Spring Force (NEED HELP QUICK!)

  1. Mar 3, 2009 #1
    1. Problem

    In the figure, we must apply a force of magnitude 63 N to hold the block stationary at x = -2.0 cm. From that position we then slowly move the block so that our force does +4.1 J of work on the spring-block system; the block is then again stationary. What is the block's position (x)? (There are two answers.)

    Here's the figure: http://www.webassign.net/hrw/7-11a.gif

    ? cm (smaller value)
    ? cm (larger value)

    2. Attempt at a solution

    +4.1 J = (1/2)k * (xf2 - xi2)
    (+63 N)î = -k(-2.0 cm)î k = 3.15 x 103 N/M
    xi= -2.0 cm

    I got some weird decimal answer which was apparently wrong.
    I also don't understand what the smaller and larger value is.
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Mar 3, 2009 #2

    Doc Al

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    Staff: Mentor

    What did you get?
    You could move the block in either direction.
     
  4. Mar 3, 2009 #3
    0.4

    So the smaller value is negative and the larger value is positive, am I right?. I tried -0.4 too. It's wrong.
     
  5. Mar 3, 2009 #4

    Doc Al

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    How did you get the answer of 0.4? (40 cm?) The equation you posted previously looked OK to me. Did you solve it?
     
  6. Mar 3, 2009 #5
    Yeah, I got xf = 0.4 cm.
     
  7. Mar 3, 2009 #6

    Doc Al

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    Redo your solution. (And check your answer by plugging back into the original equation and seeing if it works.)
     
  8. Mar 3, 2009 #7
    I got 2 this time. Am I suppose to convert that to something?
     
  9. Mar 3, 2009 #8

    Doc Al

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    This is the equation you're solving, right? When you get an answer, plug it back into the equation to verify it.

    How are you solving it? Show each step along the way.
     
  10. Mar 3, 2009 #9
    I got 2. I'm sure.

    Square root of this [4.1 / (1/2)(3.15 x 103) + (-2.0)2]
     
  11. Mar 3, 2009 #10

    Doc Al

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    Use standard units: 2.0 cm → 0.02 m.
     
  12. Mar 3, 2009 #11
    Ah crap, you're right.

    So it's...Square root of this [4.1 / (1/2)(3.15 x 103) + (-2.0 x 10-2)2]
     
  13. Mar 3, 2009 #12

    Doc Al

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    That looks better.
     
  14. Mar 3, 2009 #13
    Okay, I get 160.7. Do I have to convert that or something?
     
  15. Mar 3, 2009 #14

    Doc Al

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    Please double-check that answer. (Is 160 m a reasonable answer?)
     
  16. Mar 3, 2009 #15
    It's wrong. I got 160.
     
  17. Mar 3, 2009 #16

    Doc Al

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    Let's do it step by step.

    A = 4.1 / (1/2)(3.15 x 103) = ?

    B = (-2.0 x 10-2)2 = ?

    A + B = ?

    √(A + B) = ?
     
  18. Mar 3, 2009 #17
    A = 25830
    B = 4e-4
    A + B = 25830.0004
    ? = 160.7

    I tried 161, it's also wrong.
     
  19. Mar 3, 2009 #18

    Doc Al

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    Way off. (You're probably multiplying where you should be dividing.)

    A = 4.1 / [(1/2)(3.15 x 103)] = ?
     
  20. Mar 3, 2009 #19
    o wait a second...you're right
     
  21. Mar 3, 2009 #20
    I got .0027. Do I have to convert this?
     
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