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: Classical Mech problems help

  1. Sep 19, 2005 #1
    Does anyone know how to solves these, hints anything??? I was able to do #1,2 and got lost at 3 and without 3 I couldnt do 4 and then i couldnt do 5 :(
    PLease help

    #3 in special relativity the dynamics are different. In particular, the momentum p is no longer mv. Failure to remeber this causes a lot of trouble. THe lagrangian of one dimensional speical relativity is
    L(x, x(dot))= -mc^2 (sqroot of (1-(xdot/c)^2)-V(x)
    where c is the speed of light. Use the definition of hte momentum p= partial derviatives of (L/xdot) to express the relativistic momentum in terms of velocity.

    #4 use the lagrangian of #3 to find the hamiltonian of relativistic particle. Remember that H must be expressed as a function of p and x only. This hamiltonian is the energy of a relativistic particle.

    #5 Use the hamiltonian of problem 4 and hamilton's equations of motion to express the velocity of a relativistic particle in terms of its momentum.
  2. jcsd
  3. Sep 20, 2005 #2


    User Avatar
    Science Advisor

    For #3, the best hint is to tell you to do exactly what the problem says: You are given L(x,xdot) so you can calculate L(x,xdot)/xdot. Now find the partial derivatives of that.
    #4: I presume you know how to form the Hamiltonian of a classical partical from the Lagrangian. As the problem says, be sure you use p instead of xdot. In classical mechanics, p is just mass times xdot but in relativity you will need the formula you get in #3. Write the Hamiltonian in terms of x and xdot, solve the formula you got in #3 for xdot as a function of p, and substitute.

    #5: Put the Hamiltonian you got in #4 into Hamilton's equations and solve for the velocity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook