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: Two protons moving toward each other

  1. Sep 27, 2007 #1
    1. The problem statement, all variables and given/known data

    Two protons that are very far apart are hurled straight at each other, each with an initial kinetic energy of 0.16 MeV, where 1 mega electron volt is equal to 10^6*(1.6*10^-19) joules. What is the separation of the protons from each other when they momentarily come to a stop?

    2. Relevant equations

    U,electric=(1/4 pi epsilon)*(q1q2/r)

    3. The attempt at a solution

    I've been trying to figure this one out for about a half hour. I tried finding the potential energy U by rearranging terms in the equation for E. I then took that number and set it equal to (1/4 pi epsilon)*(q1q2/r)) and tried to find r (the distance) that way, but my answer was wrong. Any ideas?
  2. jcsd
  3. Sep 27, 2007 #2


    User Avatar
    Gold Member

    I can't work out what you did but if you set the initial kinetic energy equal to the final potential energy, it's trivial, surely ?
  4. Sep 27, 2007 #3

    Thanks...that's what I did. I took that number and then used that in the equation for electric potential energy to obtain the distance.

    Sometimes I have a habit of making silly mistakes/overthinking things.
  5. Sep 27, 2007 #4


    User Avatar
    Homework Helper

    Your description is a bit nebulous. Could you please show your numerical calculations? What result have you been getting?
  6. Mar 2, 2008 #5
    I am attempting to solve this problem. It seems simple enough but I seem to be getting the wrong answer.

    Here is what I have done:

    --------------------- = (.16)(1e6)(1.6e-19)

    2.304e-28 = 2.56e-14*r

    Can someone help me figure out where I went wrong?
  7. Mar 2, 2008 #6


    User Avatar
    Homework Helper

    Remember your initial energy is 2*(0.16 MeV), since both protons are moving.
  8. Mar 2, 2008 #7
    That was the problem. Thank you very much.
  9. Mar 7, 2008 #8
    I have this same homework question (but my Ki is .21MeV). Can someone explain it to me in detail? I'm trying to make sure I understand why it's done this way.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook