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Homework Help: Electric Force- Finding angle of Force on q1 due to q2 and q3

  1. Aug 27, 2011 #1
    1. The problem statement, all variables and given/known data
    A charge of q1=3.7mC is located at (x=345, y=(-237))cm. A charge of q2=9.1mC is located at (2.0,5.7)m. A charge of q3=7.2mC is located at ((-27.9),83)m. What is the angle of the force on q1 due to q2 and q3? (deg, relative to +x axis, three significant figures)

    2. Relevant equations
    Coulomb's Law

    3. The attempt at a solution
    Ok so the first step is to find the x,y distance between q1 and q2, and the distance between q1 and q3.

    q1 distance to q2 = (-1.45, 8.07)
    q1 distance to q3 = (-31.35, 85.37)

    so then I find the angle on q1q2 and q1q3.

    q1q2 = arctan(8.07/-1.45) = 100.186
    q1q3 = arctan(-31.35/85.37) = 110.1645

    add the angles together and I got 210.35, and the answer is wrong. Is there anything I'm missing or did something wrong?

  2. jcsd
  3. Aug 27, 2011 #2


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    You have completely disregarded the sentence marked in red in the problem.
    Find the forces of both charges q2 and q3 in vector form, applying Coulomb's Law, calculate the resultant, and find the angle the resultant force encloses with the x axis.

  4. Aug 27, 2011 #3


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    I think you have lost focus.
    The question is " What is the angle of the force on q1 due to q2 and q3?" I have highlighted that part of the question you posted

    Forces depend on the size of the charges [you haven't used them] and the direct distance apart - not some (x,y) components of that distance.
  5. Aug 27, 2011 #4


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  6. Aug 28, 2011 #5
    Thanks, it's not just I'm not focused. I'm just not that clear of the question.

    I did find out both forces which are.

    F12 = 4506.692N
    F13 = 28.99N

    But the question is how do I know which one is horizontal or vertical? I did draw the graph, but I'm just confused on how to find which ones face which.
  7. Aug 28, 2011 #6


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    Since the original charges are not arranged in a triangle with a vertical and horizontal side, neither of these forces is horizontal nor vertical.
    You now need to find the x- and y- components of each force, so you can combine them.

    The x- and y- components you found of the distances will help there.
  8. Aug 28, 2011 #7
    Thank you so much, I figured out the answer. I forgot about that. :)
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