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: Relativistic Momentum and Energy

  1. Aug 18, 2012 #1
    Hello guys,

    The question is: What is the momentum, in conventional SI units, of a proton of momentum 685 MeVc-1?

    So, I tried two methods which yielded slightly different answers, both proximate to the actual answer (3.66 x 10-19)

    What I first tried to do was to rearrange: ρ=γm0v

    To find the particles velocity and then multiply it by the proton's rest mass in kg. I got 2.995 x 10-19 kgms-1

    The second method, which gave me the result 3.45 x 10-19 kgms-1 was the following:

    From: E2 = (m0)2c4 + p2c2 I got a value for total energy. I then subtracted the rest energy from that and I got a value for KE.

    I equalled that to p2/2m and solved for p (using m in kg and the energy converted from MeV to J)

    Are my answers wrong? The book says 3.66 x 10-19 and I simply can't get that!

    Thanks in advance!
  2. jcsd
  3. Aug 18, 2012 #2
    the book answer is correct.

    a question for you : What does MeVc-1 mean?

    can you do a dimension analysis on MeVc-1
  4. Aug 18, 2012 #3
    Oh, got it! Thanks! I got the number!
    Last edited: Aug 18, 2012
  5. Aug 18, 2012 #4
    Oh, and, if you don't mind, could you (or anyone else) help me with the following problem please?

    A proton initially at rest finds itself in a region of uniform electric field of magnitude 5.0 x 106 Vm-1. The electric field accelerates the proton for a distance of 1 km.

    Find the kinetic energy of the proton.

    So, what I did was the following:

    KE = q * E * s

    I then converted the result from J to MeV. I, however, get 5000 MeV and the book gets 500 MeV. Is my line of thought incorrect?

    Thanks once again!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook