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Simple degree help (tan-1 fy/fx)

  1. Feb 18, 2007 #1
    1. The problem statement, all variables and given/known data

    Three point charges are located at the corners of an equilateral triangle as shown in Figure P15.13 (q = 3.00 µC, L = 0.400 m). Calculate the net electric force on the 7.00 µC charge.

    ° (counterclockwise from the +x axis)

    http://www.webassign.net/pse/p23-07alt.gif

    3. The attempt at a solution

    F_12 = 8.9875 x 10^9 (3e-6)(7e-6)/.4e2 = 1.18 N

    F_13 = 8.9875e9 (7e-6)(4e-6)/.4e2 = 1.57 N

    then

    F_12x = 1.18cos60 = .59 N
    F_12y = 1.18sin60 = 1.0219 N
    F_13x = 1.57cos60 = .785 N
    F_12y = -1.57sin60 = -1.3697 N

    then i added them up

    Fx = .59 + .785 = 1.375 N
    Fy = -1.3597 + 1.0219 = - .3378 N

    I know that to get the degree its arctan Fy/Fx but i get the incorrect answer. Is there something wrong in the equation or im i using the wrong number?

    I understand how to do the first part which is 1.4 N of electric force on 7uC charge.

    Thanks for any help.
     
  2. jcsd
  3. Feb 18, 2007 #2

    hage567

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    What angle are you getting??
     
  4. Feb 18, 2007 #3
    i get -13.80*

    if i add 180 to that i get 166.2*

    both are incorrect.
     
  5. Feb 18, 2007 #4

    hage567

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    Homework Helper

    Wouldn't you want to add 360 to it? That would give you the angle from the x-axis going counterclockwise instead of clockwise. Just a guess.
     
    Last edited: Feb 18, 2007
  6. Feb 18, 2007 #5
    You are correct. it is adding 360.

    I thought it was 180 because i was thinking of a triangle which is 180*.

    Thanks again.
     
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