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!

A tricky differential equation

  1. Mar 9, 2004 #1
    Hello :smile:

    I have a differential equations, which I don't know how to solve. Maybe somebody has a tip or two

    The equation is:
    d^2 m / d x^2 = -3x
  2. jcsd
  3. Mar 9, 2004 #2


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    So m is a function of x and its second derivative with respect to x is -3x? What function gives ax when differentiated? And then what function gives THAT function when differentiated? Do you have any initial conditions?
  4. Mar 9, 2004 #3
    Well, I need to find the complete solution. All I know is that I have a function m(x) and that the interval of x is from 0 to 4.
  5. Mar 9, 2004 #4

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    what selfadjoint was getting at was that this is a very easy separable solution, though you've not met those words yet perhaps.

    suppose y' = 2x, ( ' means diff wrt x)

    y= x^2+c

    suppose z'' = y' in the above,
    then z' = x^2+c,

    so what is z?
  6. Mar 13, 2004 #5
    I haven't taken diff. eq. in a while, so I might be wrong, but isn't that like saying:

    m" = -3x ?

    then just integrate: (I'm using | as the symbol for integral)

    |(m" dm) = |(-3x dx)
    m' = -3/2 x^2 + C

    integrate again, gives:
    |(m' dm) = | (-3/2 x^2 + C dx)
    m = -1/2 x^3 + Cx + D

    That's it!

    I don't know what the interval from 0 to 4 falls-in though....
  7. Mar 24, 2004 #6
    Well, wouldn't the interval be just a definite integral from 0 to 4? I may be wrong, if I misread your question. [b(]
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: A tricky differential equation
  1. Differential equation (Replies: 3)

  2. Differential equation (Replies: 2)

  3. Differential equation (Replies: 2)