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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

    E=mc^2+K+U
    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

    Mentz114

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    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

    dynamicsolo

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    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:
    Uf=Ki

    9e9*(1.6e-19)2
    --------------------- = (.16)(1e6)(1.6e-19)
    r

    2.304e-28 = 2.56e-14*r
    r=9.0e-15

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

    hage567

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    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.
     
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