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: Total energy, mass, and momentum.

  1. May 30, 2005 #1
    An electron with speed 0.8c, has a rest energy of .51MeV.

    What are its mass, its momentum, and its enegy (including rest energy)?


    ----------------------------------------------------------------------

    Ok, isnt it possible to get the three answers without using the .5MeV ?

    I have 3 formulas for mass, momentum and total energy that dont use the .5Mev


    but then I feel like I might be doing something wrong since they gave me the value.



    Im not sure how to post the forumlas, but they are on this page

    http://galileo.phys.virginia.edu/classes/252/energy_p_reln.html



    thanks for any help
     
  2. jcsd
  3. May 30, 2005 #2
    The rest energy is the [tex]m_0c^2[/tex] part of the formulas found on that page. Also, hyperphysics ( http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html ) is the #1 most helpful site I have found when I need to reference something. You will probably understand the formulas a little better if you look at the sections dedicated to relativistic energy there.
     
    Last edited by a moderator: May 30, 2005
  4. May 30, 2005 #3

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    The only way to avoid using the given information is if you know the rest mass of the electron. Look at the E equation in the limit of zero velocity. What is it telling you?

    OOPs too late again. It's still the right question, and now you have the answer :smile:
     
  5. May 30, 2005 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    You don't need that 0.51 MeV.Just that

    m_{el}~9.1*10^{-31}Kg.

    Daniel.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook