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Homework Help: Resultant force formula

  1. Aug 4, 2007 #1
    The problem statement, all variables and given/known data

    1. A charge A of -5 µC has two nearby charges: +3.0 µC at a distance 0.3 m vertically above it, and +4.0 µC at a distance of 0.4 m horizontally. Find the magnitude of the force in newton on charge A. Hint: find the vertical and horizontal components, then add these vectors to obtain the resultant.

    2. Two charges of +1 and +36 nanocoulomb are separated by a distance of 100 mm. A 3rd test charge is moved along the line between these two charges. Where is the test charge when it experiences a net force = zero?

    Give your answer in mm measured from the first of the two charges. Hint: think about the ratio of the two charges and deduce a ratio for the distances.
    The attempt at a solution

    1. Using the formula |F| = k[(|q1||q2|)/r^2)]

    I found Fhorizontal = 1.12*10^-6
    and Fverticle = 1.5*10^-6

    I then use pythag, resultant = sqrt( (F_horizontal)^2 + (F_vertical)^2 )
    to give answer of 1.87*10^-6.
    I think it may be right, can somebody verify?

    2. I'm not sure how to do this one!
    Last edited: Aug 5, 2007
  2. jcsd
  3. Aug 5, 2007 #2
    1. Havent checked the values, but the method looks right.

    2. Let the distance from the charges at the point where Fnet is zero be (100-x) and x in mm. Now, find the force exerted by each of the charges on your moving charge and equate them. You should get a quadratic in x. Solve for x.
  4. Aug 6, 2007 #3
    Managed to do those ones, but I've got a new question!

    Calculate the resultant force in newton on the left hand charge q = 12 µC due to charges q = -3 µC at distance 0.1 m to the right and a charge 13 µC at a further distance 0.2 m to the right (measured from the 2nd charge).
    Use the correct sign, taking force to the right as positive and to the left as negative.
    12 µC....................q =-3 µC.................................... 13 µC

    |<------ 0.1 m ----->|<------------ 0.2 m-------------->|

    A step by step guide would be very handy! Thanks in advance to anybody who can help
  5. Aug 6, 2007 #4
    Give it a shot first. Use coulombs law and make a FBD to point yourself in the right direction.
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