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Homework Help: Find the distance that a force stretches a spring.

  1. Apr 17, 2013 #1
    1. The problem statement, all variables and given/known data

    A mass of 6.40 kg is hung from a spring with aforce constant of 7.80*10^3 N/m. The spring unstretched is 0.120m long. Find the distance that the spring stretches.

    2. Relevant equations

    F = k*x is the way that gives the answer in my textbook.

    I tried W = 0.5*k*x^2.

    3. The attempt at a solution

    What is wrong with the following? I used W = 0.5*k*x^2 = 0.5*7800N/m*x^2 . Then made W = m*g*x. So, m*g*x = 0.5*7800N/m*x^2. Then I cancelled out the x from both sides, and isolate the remaining x, so x = (m*g)/(0.5*78000N/m) = 0.136m. My answer is close to the correct answer of x = 0.128m, but I would really like to know what I don't understand which led me to the wrong answer.
  2. jcsd
  3. Apr 17, 2013 #2
    Use Newton's 2nd law. Draw your FBD and correctly set up your terms for ƩF, knowing that the system is at static equilibirum. You'll be able to find the change in x of the spring from its unstretched length this way.

    The x in your 'mgx' term (which in this case is also the gravitational potential energy) isn't the same as the x in your '.5mgx^2' term. Energy methods aren't always so convenient in these types of problems.
  4. Apr 17, 2013 #3


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    Homework Helper

    hi student34! :smile:
    you're using conservation of energy, which (correctly) gives you the lowest point of the motion if the mass is released from rest in the unstretched position, and allowed to oscillate up and down

    but the question is about carefully lowering the mass until the system is in equilibrium :wink:
  5. Apr 17, 2013 #4
    Thanks for both of these answers!!!
  6. Apr 17, 2013 #5
    If I know the spring constant, I can calculate the displacement given the mass,

    This must be wrong if the text book wants exactly .008m as displacement.
    So I fail. A for effort.
  7. Apr 17, 2013 #6
    ...and the units in the last expression all must be labeled N/m.
    I am a disgrace to the profession.
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