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

Homework Help: Gamma function application

  1. Oct 19, 2006 #1
    A particle of mass m starting from rest at x=1 moves along the x axis toward the origin. Its potential energy is [itex] V=\frac{1}{2}mlnx[/itex]. Write the Lagrange equation and integrate it to find the time required for the particle to reach the origin.

    Lagrange Equation in 1-D:
    [tex]\frac{d}{dt}\frac{\partial L}{\partial\dot{x}}-\frac{\partial L}{\partial x}=0[/tex]
    [tex]L = T - V = \frac{1}{2}mv^{2}-\frac{1}{2}mlnx =\frac{1}{2}m\dot{x}^{2}-\frac{1}{2}mlnx [/tex]

    Substitute L in Lagrange Equation:
    [tex]\frac{d}{dt}\frac{\partial}{\partial\dot{x}}\left(\frac{1}{2}m\dot{x}^{2}\right)-\frac{\partial}{\partial x}\left(-\frac{1}{2}mlnx\right)=0[/tex]
    [tex]\frac{d}{dt}\frac{\partial}{\partial\dot{x}}\left(\frac{1}{2}m\dot{x}^{2}\right)=\frac{\partial}{\partial x}\left(-\frac{1}{2}mlnx\right)[/tex]

    ... And I don't really know what to do from here. The answer is given and it is supposed to be [itex]\Gamma\left(\frac{1}{2}\right)[/itex]. Can someone tell me where to go from where I left off? Thank you!
  2. jcsd
  3. Oct 19, 2006 #2
    i don't know if this will help... but you can do this problem without using the Lagrangian. since the motion is in one dimension, energy is always conserved.

    then integrate through dt=dx/v, express v as a function of x using energy. then integrate both side from x=1 to x=0
  4. Oct 19, 2006 #3
    Well, the question says specifically to use the Lagrange Equation. My problem is I don't know what to integrate and what I'm integrating over. Also, I'm assuming I have to use either one of these relations:

    [tex] \Gamma\left(p\right)=\int^{\infty}_{0}x^{p-1}e^{-x}dx [/tex]


    [tex] \Gamma\left(p+1\right) = p\Gamma\left(p\right) [/tex]
  5. Oct 20, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper

    I think there is another form that will help you. See


    equation (5)



    equation (1)

    Your equation


    can be integrated by separating variables



    [tex]2vdv =-\frac{dx}{x}[/tex]

    Integrate this and set v = 0 at x = 1 and then separate variables and integrate again. That will give you that Euler form for [itex]\Gamma\left(\frac{1}{2}\right)[/itex]
    Last edited: Oct 20, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook