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Homework Help: Coulombs law and vectors, find the force vector

  1. Aug 17, 2009 #1
    This is probably really obvious but I have a hard time with vectors and really don't know what it's supposed to look like..I'm not even sure if to draw the grav. force vectors starting from the ends of the ropes or straight down from the centers of the bullets.

    1. The problem statement, all variables and given/known data

    Two identical bullets, which both weigh 500 mg are hanging off of ropes 40 cms in length. They were both given equal charges of the same type (both positive or both negative) after which they repelled from each other and remained fixed at a position such as to form a 60 degree angle between the ends of the ropes. Find the charges of the bullets and the force that tightens the ropes.

    2. Relevant equations

    Coulombs law- F= k times q1q2/r2

    3. The attempt at a solution

    Ok so I drew a triangle and by using trig laws I found that r= 34 cm=0.34 m. Also the grav. force on the two bullets is 0.00049 N. So now I should be able to find the force between the bullets from the grav. vectors and the angle and then solve for the charges except I have no idea as to how to do that? Also my drawing is attached.

    Attached Files:

  2. jcsd
  3. Aug 17, 2009 #2

    Doc Al

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    Staff: Mentor

    Pick one of the bullets and identify all the forces acting on it. (Hint: Three forces act.) Draw a free body diagram for that bullet. What does the fact that the bullet is in equilibrium tell you about the net force?
  4. Aug 17, 2009 #3
    Hi there,

    Nice diagram. You are in a static problem, meaning nothing moves. Therefore, from Newton's second law, you can deduce what??? (come on that's an easy one)

    From this deduction, you should be able to draw an equation.

  5. Aug 18, 2009 #4
    I get that the sum of all the forces is zero. However I still have two unknown forces, one along the rope and the electromagnetic force. Even knowing the sum of all three doesn't help. What am I missing?
  6. Aug 18, 2009 #5

    Doc Al

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    Staff: Mentor

    The electric force can be expressed in terms of the charge, so your only unknowns end up being charge and tension. Which is a good thing, since they are just what you are asked to find.

    Hint: Set up two equations: one for vertical forces, one for horizontal.
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