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Homework Help: The mass of a proton is 1836 times the mass of an electron.

  1. Mar 12, 2012 #1
    1. The problem statement, all variables and given/known data

    The mass of a proton is 1836 times the mass of an electron.

    A) A proton is traveling at speed . At what speed (in terms of ) would an electron have the same kinetic energy as the proton?

    B) An electron has kinetic energy K . If a proton has the same speed as the electron, what is its kinetic energy (in terms of K )?

    2. Relevant equations

    3. The attempt at a solution

    The answer for A is 42.85, and to get it here is what I did.





    Part B the answer is 1836

    But I have no clue on how to get it. Since their velocities are equal I was thinking of setting them both to 1.


    I don't know what to do after this.. tried solving for M but that doesn't make sense.

    Thank you
  2. jcsd
  3. Mar 12, 2012 #2


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    Science Advisor
    Homework Helper

    Hi Mdhiggenz! :smile:

    Where's the difficulty? :confused:

    Just call the velocity "v", write out the KE for both particles, and find their ratio …

    what do you get? :smile:
  4. Mar 12, 2012 #3

    As i see from your previous posts you have a little trouble in solving equations :-)
    Don't worry it will go away with practice.

    Write the KE=(mv*2)/2

    so v^2=2KE/m

    now since velocity for proton and electron is given equal we have
    2KE(p)/m(p) = 2KE(e)/m(e) (here KE(p) means kinetic energy of proton)

    now you have a relation between m(p) and m(e) so equate the two equations above.
  5. Mar 12, 2012 #4
    I got Ke(p) = 2ke/me*mp is that correct?
  6. Mar 12, 2012 #5
    No, the factor of 2 is extra.
    RHS should not have the factor 2.

    As i have written 2KE(p)/m(p) = 2KE(e)/m(e)

    i.e same as KE(p) = KE(e)/m(e) *m(p)
  7. Mar 12, 2012 #6
    Thx email I finally got it. I was missing the mp=1836me relation ship and when you plug and chug using the ration is all makes sense.
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