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Superposition of 3 forces on a point charge

  1. Apr 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Four charges are placed on the corners of a rectangle. What is the resultant force on the positive charge (a = 1.5 m, b = 0.5 m, q = 1.1 × 10-9C)?
    (see attached drawing)

    2. Relevant equations
    Im having problems summing the forces using superposition

    3. The attempt at a solution
    F = k(qq) / r^2

    F1 = 4e-9 (r=1.5m)
    F2 = 4e-8 (r=0.5m)
    F3 = 4e-9 (r=1.58m)

    then when i try to superimpose i get probs..
    i put tail of F1 vector at origin, then put tail of F2 vector ontop tip of F1.
    I do the same for F3.
    then i draw line from tail of F1 to tip of F3.
    this is equal to 2*F3.
    = wrong

    please help. it has to be something with doing superposition wrong i think??
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Apr 23, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    I'll suggest figuring out the horizontal and vertical component of each vector (using sine and cosine), and find the resultant by adding up components.
  4. Apr 23, 2009 #3


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    Staff: Mentor

    What is the direction of each of these vectors?

    Also, you should check your arithmetic and keep more digits until you reach the final answers. For F1 I get 4.84e-9 which is a big difference!

    Always keep more digits during the intermediate steps, until you reach your final answer, and only then round it off to the proper number of digits.
    Last edited: Apr 23, 2009
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