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Piston Question: Have solution, don't understand rationale

  1. Nov 5, 2014 #1
    1. The problem statement, all variables and given/known data
    The drawing below shows a hydraulic chamber in which a spring (spring constant = 1580 N/m) is attached to the input piston (A1 = 15.3 cm2), and a rock of mass 41.9 kg rests on the output plunger (A2 = 66.6 cm2). The piston and plunger are nearly at the same height, and each has a negligible mass. By how much is the spring compressed from its unstrained position?

    2. Relevant equations

    F1/A1=F2/A2
    F=kx
    3. The attempt at a solution
    F1/15.3 cm^2= 410.62 N/66.6 cm^2 F1=94.33 N

    94.33 N= 1580 N/m
    0.0597 m

    I'm pretty sure this is the correct solution, I just don't understand why I should use F=kx rather than F=kx^2.
     
  2. jcsd
  3. Nov 5, 2014 #2

    berkeman

    User Avatar

    Staff: Mentor

    It looks like you are confusing the equation for spring force (F=-kx) with potential energy stored in a spring (E=1/2kx^2) :-)

    http://en.wikipedia.org/wiki/Spring_(device)

    http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html

    .
     
  4. Nov 5, 2014 #3
    Oh, I sure was. Thank you so much!
     
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