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Homework Help: Leg Presses Work Energy Theorem Confusion

  1. Nov 22, 2012 #1
    1. The problem statement, all variables and given/known data

    As part of your daily workout, you lie on
    your back and push with your feet against a platform attached to
    two stiff springs arranged side by side so that they are parallel to
    each other. When you push the platform, you compress the springs.
    You do 80.0 J of work when you compress the springs 0.200 m
    from their uncompressed length. (a) What magnitude of force must
    you apply to hold the platform in this position? (b) How much
    additional work must you do to move the platform 0.200 m
    farther, and what maximum force must you apply?

    2. Relevant equations

    Work Energy theorem ONLY!

    3. The attempt at a solution
    I assumed that the 80 J would be equal to the sum of work done to compress each spring... So 80 J = 2* (.5 K x^2) ... However, the book had an attempt directly the 80 J = .5 Kx^2... Kindly Clarify why is it so if the force is applied on two springs??? THANKS A LOT!
  2. jcsd
  3. Nov 22, 2012 #2


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    Homework Helper
    Gold Member
    2017 Award

    Perhaps they are letting K represent the effective spring constant of both springs together. Your approach is also fine. Both methods should give the same answers for (a) and (b).
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