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Homework Help: Electrostatic force

  1. Jun 30, 2009 #1
    1. The problem statement, all variables and given/known data
    A -0.7nC charge is located at the origin, while a 1.9nC charge is located 3.9 m away along the x-axis and a 0.8nC charge is located -1.7m away along the y-axis. The Coulomb constant is 8.98755*10^9 Nm^2/C^2. Find the magnitude of the electrostatic force on the charge at the origin. Answer in units of nN.
    What is the angle between the electrostatic force on the charge at the origin and the positive x-direction? Answer in degrees as an angle between -180 and 180 measured from the positive x-axis, with counterclockwise positive. Answer in units of degrees.

    2. Relevant equations

    3. The attempt at a solution
    I used the equation F=k(Q/r^2) to find the force of the charge between the origin and each of the charges. For the force along the y-axis, I got 2.960604706e-9 and -3.064984998 along the x-axis. However, I'm not sure where to go from there.
  2. jcsd
  3. Jun 30, 2009 #2


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    Hi peaceandlove! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    You have two forces, so now all you have to do is add them.

    Forces are vectors, so you use the vector law of addition …

    in other words, you can add them by using a vector triangle, or you can just add the components (same result) …

    since the two forces in this case are perpendicular, either method should be extremely easy. :wink:
  4. Jun 30, 2009 #3
    What do you mean by add the component?
  5. Jul 1, 2009 #4


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    (just got up … :zzz:)

    If F has components Fx and Fy, and G has components Gx and Gy, then vector addition means you add the components:

    if H = F + G, then H has components Fx + Gx and Fy + Gy :smile:
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