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Homework Help: Finding net force on charges on xy plane

  1. Sep 21, 2008 #1
    1. The problem statement, all variables and given/known data

    A proton is on the x axis at x = 1.2 nm. An electron is on the y axis at y = 1.2 nm.
    Find the net force the two exert on a helium nucleus (charge +2e) at the origin.
    Express your answer using two significant figures. Enter your answers numerically separated by a comma, such that " Fx, Fy = ...,..."

    2. Relevant equations

    electron charge, e = -1.6*10^-19 coulombs
    proton charge, + e = + 1.6*10^-19 coulombs
    distance, r = 1.2 nm = 1.2*10^-9 m
    constant, k = 9*10^9

    force in newtons, F = kq_1q_2/r^2 where q_1 and q_2 represent point charge

    3. The attempt at a solution

    charge at origin = +2e = 3.2*10^-19 coulombs

    Fy = force y axis = electron and origin charge = [(9*10^9)(-1.6*10^-19)(3.2*10^-19)]/(1.2*10^-9)2 = - 3.2*10^-10 newtons

    Fx = force y axis = proton and origin charge = [(9*10^9)(+1.6*10^-19)(3.2*10^-19)]/(1.2*10^-9)2 = - 3.2*10^-10 newtons

    Fnet = sqrt(Fy^2 + Fx^2) = sqrt(2.048*10^-19) = 4.5255*10^-10 newtons

    i entered these and they were wrong:

    Fx, Fy = 4.53*10^-10, -4.53*10^-10
    Fx, Fy = 3.2*10^-10, -3.2*10^-10

    are my calculations correct? what should i enter?
  2. jcsd
  3. Sep 22, 2008 #2
    yea, i solved that problem the same way. the problem is in the book, and using that same equation for Fx, Fy i got the same answer as the solution in the book. but when i did the problem on mp (my numbers were different) the answer was wrong - and i solved it the SAME way. aghhh im sooo lost. i dont know why its wrong =/
  4. Sep 22, 2008 #3
    i originally had the signs wrong, Fx is actually -3.2*10^-10, my Fy is wrong, it is a positive value though. why is it wrong, i thought the two forces would be equal in magnitude.

    it says my Fy value is "you may have made a rounding error or used the wrong number of significant figures." it is currently Fy = 3.2*10^-10

    could someone please help me for Fy
  5. Sep 22, 2008 #4
    i would have thought it would have been the same in your problem, just opposite signs, since your distances are the same. mine are different for the x-axis and y-axis, so i dont know - my answer's not even right =( im finishing up mp tomorrow afternoon though - kinda late - but im sure i'll get the rest of it by then, if ur still working on it
  6. Sep 22, 2008 #5
    i tried to run the calculations again and i am still getting 3.2*10^-10, is there any reason why the forces should have different magnitudes? it doesn't seem to make sense
  7. Sep 22, 2008 #6
    ok, i tried my answer and it keeps coming out wrong - i dont think i messed up on any of my calculations...
    i had 1.7 nm distance on the x-axis for the proton, and 1.3 nm distance on the y-axis for the electron. everything else is the same.
    my equations were:

    Fx= -k (e)(2e)/(1.7*10-9)2 = -(9*109)(1.6*10-19)(3.2*10-19)/(2.89*10-18) = -(4.608*10-28)/(2.89*10-18) = -1.594*10-10 N

    Fx= k (e)(2e)/(1.3*10-9)2 = (9*109)(1.6*10-19)(3.2*10-19)/(1.69*10-18) = (4.608*10-28)/(1.69*10-18) = 2.726*10-10 N

    i solved the problem in the book the same way, with their numbers and it was right.... but i dont know what i did wrong here...? mp marked my answers wrong with no suggestions or anything - so its like they're completely off. any idea why?
    Last edited: Sep 22, 2008
  8. Sep 23, 2008 #7
    i think your answer is right - MP had the wong answers. she's making it a practice problem now so we don't lose points.
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