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Homework Help: Simple Spring Question (Hooke's Law)

  1. Oct 12, 2009 #1
    1. The problem statement, all variables and given/known data
    The left end of a spring is attached to a wall. When Bob pulls on the right end with a 200N force, he stretches the spring by 20cm. The same spring is then used for a tug-of-war between Bob and Carlos. Each pulls on his end of the spring with a 200N force.

    How far does Carlos's end of the spring move?

    2. Relevant equations
    Hooke's Law, Newton's Second Law

    3. The attempt at a solution
    Using the fact that the spring pushes back with a -200N force, I found the spring constant to be:

    F(spring) = k * s => -200N = -k * 0.2m => k = 1000N/m

    However, I have no idea what to do with this information. When Carlos and Bob have their tug-of-war, the net force should be 0 on the spring, which leads me to believe Carlos's end of the spring is going nowhere (0cm). I tried this answer, however it turns out that it is wrong.

    I know this problem cannot be as hard as I am making it...so any help would be greatly appreciated.
  2. jcsd
  3. Oct 13, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Is there any difference in the total stretch of the spring in the two scenarios? (Consider the tension in the spring.) What is different in the two scenarios?
  4. Oct 13, 2009 #3
    After looking at this some more, it seems that the only difference is the wall isn't pulling back whereas Carlos is. So when Bob pulls on the wall, the spring stretches 20 cm, and when Carlos and Bob both pull, the spring will go 10cm each way, as they are both pulling with equal force which means the spring would have to still go 20cm total. Thanks for the reply, I knew I had to be overlooking some minute detail.
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