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Homework Help: Electric force problem

  1. Feb 12, 2010 #1
    1. The problem statement, all variables and given/known data
    Hello forums! I'd like to ask you for your assistance on this particular problem:

    Three charges are placed at the edges of a triangle (Q1=+2 C, Q2=-4 C, Q3=+3 C) as shown on the picture (b=10 cm). What is the resultant force on charge Q2 (neglecting gravity and assuming the charges are in a vacuum)?

    2. Relevant equations


    k=9*10^9 N m^2 C^-2

    3. The attempt at a solution
    The distance "a" is given by:
    tan 30°=a/b
    Which gives: a=5.7735 cm.

    Using the Pythagorean theorem I got: c=11.547 cm.

    Then I calculated the force between Q1 and Q2 to be: F1=-2.16*10^13 N
    Then I calculated the force between Q3 and Q2 and got: F2=-8.1*10^12 N

    The problem I'm having is calculating the resultant force (Fr) on Q2. Which sort of formula should I use to calculate it?
    I made a force diagram as well:
  2. jcsd
  3. Feb 12, 2010 #2
    Force is a vector so you need to add the components. Find F2y and add it to F1 and then find F2x.
  4. Feb 12, 2010 #3
    I think I got it. So, what I did was this:
    sin 30°=Fy1/F2
    From this, I got: Fy1=-4.05*10^12 N
    then I added this to F1 and got: Fy2=Fy1+F1 => Fy2=-2.565*10^13 N
    I got the x component from:
    cos 30°=Fx/F2
    And got: Fx=-7.0148*10^12 N
    Then I used the Pythagorean theorem to find the resultant and got: Fr=[tex]\sqrt{(Fy2)^2+(Fx)^2}[/tex]
    Fr=-2.65919*10^13 N

    Does this look correct?
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