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Homework Help: Direction and magnitude question again

  1. Sep 9, 2007 #1
    1. The problem statement, all variables and given/known data
    A charge of -6mC is placed at each corner of a square .1m on a side. Determine the magnitude and direction of the force on each charge.
    upper left is 1, bottom left is 2 upper right is 4 bottom right is 3
    http://img208.imageshack.us/img208/7318/payxhbx6.jpg [Broken]

    2. Relevant equations
    F=k (Q1Q2)/r^2

    3. The attempt at a solution
    sigh im back again..
    I start with F12 so i did:
    9 X 10^9(6 X 10^-3 X 6 X 10^-3/.1^2) and i got
    3.24 X 10^7

    since all the charges are the same the answer is going to be the same for all the charges so F14 is also 3.24 X 10^7.

    Now i break F12 into x and y components:
    since 1 and 2 is repulsive there is no x component so the y component is is +3.24 X 10^7N

    as for F14 there is no y component so x is -3.24 X 10^7.

    now i do resultant formula:
    total x is -3.24 X 10^7 and total y is +3.24 X 10^7
    so the resultant should be... 4.58 X 10^7 right the book get the answer as 2.96 X 10^7

    where did i go wrong?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 9, 2007 #2


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    Homework Helper

    You didn't do F13.
  4. Sep 9, 2007 #3
    F13 affects it too??!! wow how did i miss that ok so i do F13 is the same thing but the x and y comps are different:
    3.24 X 10^7cos45=2.3 X 10^7
    3.24 X 10^7sin45=2.3 X 10^7

    the x component is negative and the y is postive since its moving north west..
    now total x is -5.72 X 10^7 and total y is 5.53 X 10^7 then i do resultant formula and i get... 7.95 X 10^7 argghh what did i do wrong books answer is 2.96
  5. Sep 9, 2007 #4


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    Homework Helper

    For F13 the total force is not 3.24x10^7... because the r isn't 0.1.

    Still the force is going to be higher... does it say 2.96x10^7, or 2.96x10^8?
  6. Sep 9, 2007 #5
    its 10^7
    and i did the F13 over:
    9 X 10^9(6 X 10^-3 X 6 X 10^-3/.14^2) and i get 1.63 X 10^7
    find the component:
    1.63 X 10^7cos45=1.15 X 10^7
    1.63 X 10^7sin45=1.15 X 10^7

    now the total X is
    -1.15X10^7 + 0 + -3.24X10^7 = -4.39 X 10^7

    total y is
    1.15X10^7 + 0 + 3.24X10^7= 4.39 X 10^7
    making the resultant:
    6.2 X 10^7 ... why cant i get this thing!
  7. Sep 9, 2007 #6


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    Homework Helper

    You sure the question is exactly as you've posted it? I'm getting the same answer as you.
    Last edited: Sep 9, 2007
  8. Sep 9, 2007 #7
    physics is making go mad im sorry... the NE and SW charges are positve...so let me try this again..
    F12 is 3.24 X 10^7
    x comp:0
    y comp:-3.24 X 10^7

    F14 is 3.24 X10^7
    x comp:+3.24 X 10^7
    y comp:0

    F13 is 1.63 X 10^7
    x comp:-1.15 X 10^7
    y comp:1.15 X 10^7

    Total X is 2.09 X 10^7
    Total Y is -2.06 X 10^7

    Resultant is 2.96 X 10^7(Finally got it)
    and angle is found by doing tan inverse total y/total x and that is -45 deg
    subtract that from 360 give me 315 degs pointing towards middle of square!

    thanks for your patience lol see you tomorrow night most likely!
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