1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Rutherford scattering question

  1. Aug 31, 2006 #1

    d(chi)/dt =l/(mr^2), I don't see how to get there

    if you use the definition of angular momentum, you get l=r x mv(vectors where appropriate)

    so you can write v as v=dr/dt*r + rdx/dt*x where x is the symbol chi(close enough)and that r and x at the end of the two expressions are unit vectors

    So you can write the magnitude of l=|mr*v| where v is as above. Then I'm stumped
  2. jcsd
  3. Sep 1, 2006 #2


    User Avatar
    Homework Helper

    Only the tangential part of v contributes to L ... it is ANGULAR momentum.
    the radial part has r x (dr/dt)r_hat = 0 .
    So, with the angular part, as you say, L = r m r dtheta/dt
    => dtheta/dt = L/mr^2 .
  4. Sep 1, 2006 #3
    Okokok I think you cleared it up

    So to write it out completely, L=mr x (dr/dtr_hat+rdx/dtx_hat)

    There's a distributive property for the cross product, right? So you get mr x dr/dtr_hat, and r and r_hat are parallel so it's 0, and you're left with L=mr x r dx/dtx_hat, the magnitude of which is mr^2dx/dt?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook