Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help

  1. Jul 14, 2005 #1
    A compressed spring that obeys Hooke's law has a potential energy of 18 J . If the spring constant of the spring is 400 N/m, find the distance by which the sping is compressed.

    Please correct me if I am wrong, I'm not sure how find the distace.
    My work: k 400 N/m x=18J = 400 N/m/18 J = 0.045

    But I think my answer has to be in cm, or m. please help.
  2. jcsd
  3. Jul 14, 2005 #2


    User Avatar
    Science Advisor

    For a constant force, work is force times distance. For a variable force, such as the spring force, here 400x where x is the distance compressed, work is the integral of force times distance: [tex]\int 400x dx[/tex]= 200x2. Solve the equation
    200x2= 18.
    (Since the spring constant is given as 400 N/m, your answer will be in meters.)
  4. Jul 14, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you are at a pre-calc level, then what you need to know is the expression for the elastic potential energy stored in a spring that is extended/compressed through a distance 'x' from equilibrium. This is given by, [itex]PE = 0.5~ kx^2 [/itex] , where 'k' is the spring constant.
  5. Jul 14, 2005 #4
    It is not pre-cal, the spring is compressed that obeys Hooke's law. Thanks for the quick responses. Goku I understand what you are saying. But Halls so Ivy I did not understand how you got 200x*2= 18
  6. Jul 14, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    Using the power law. The derivative of 200x^2 is:

    [tex]\frac{d}{dx}200x^2 = 2 * 200x^{2-1} = 400x[/tex]

    The integral, or anti-derivative, of 400x = [tex]200x^2[/tex]

    I think Gokul was asking if you'd taken calculus or not.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook