1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
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

    HallsofIvy

    User Avatar
    Staff Emeritus
    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

    Gokul43201

    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

    BobG

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Need help
  1. Need HELP! (Replies: 8)

  2. Need Help! (Replies: 1)

Loading...