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Homework Help: Electric Force Vector Problem

  1. Jul 26, 2012 #1
    1. The problem statement, all variables and given/known data

    http://postimage.org/image/y4h1ubp8b/ [Broken]

    2. Relevant equations


    3. The attempt at a solution

    This is a problem involving Electric Force:

    http://postimage.org/image/y4h1ubp8b/ [Broken]

    My goal is to find the direction and magnitude of q1. However, every time i do it, i get .04 N @ 189 degrees when our teacher says it is 262.

    I am also confused about what angle i should use when finding the components on q1 and q3. Is it 64 degrees or 26 degrees? Can someone show me what angle i should use and why?

    I have a big exam tommorow and i am very stressed. I'd love an answer.

    The Forces are as follows: F12=.054, F13=.014, F23= .11

    Here is some work: sin(26.6)*.014= .006, which is x component of vector

    cos(26.6) *.014- .054=-.41, which is my y component

    I do phythag to get my force, which is .41 Newtons.

    When I do inverse tangent of .006/.041, i get 8 degrees. I add this to 180 to get a final answer of 188. My teacher said its near 262. What did i do???????
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 26, 2012 #2


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    Do you mean 262 N, or 262° ?

    q1 doesn't have a direction.

    Do you mean the direction of the force on q1?

    Last edited by a moderator: May 6, 2017
  4. Jul 26, 2012 #3
    One helpful suggestion: people will be more willing to help you if you take the time to write out everything properly. Throughout your question, you've ignored units in most places (the cardinal sin of physics!), dropped zeroes (do you mean 0.41N or 0.041N?), and not explained the conventions you're using. "@ 189 degrees" doesn't mean anything to us. 189/262 degrees relative to what? And in which direction, clockwise or counter-clockwise? Like SammyS said, q1 doesn't have direction. Do you mean the force on q1? As it is, you've made it very difficult for someone to help you because you haven't taken the time the write your question out clearly. I think I've mostly been able to figure out what you mean, and I'll help you—but honestly, I'm not sure I should be doing so. I'm doing it since you have a test tomorrow, but please don't let this reinforce this kind of carelessness.

    You've calculated the magnitude of the force correctly: it is indeed 0.04 Newtons (mind your significant figures!). You haven't shown which angle you're labeling with 26.6 degrees, but your components are correct (with the exception of the missing zero in your y-component). Thus, [itex]arctan(0.041N/0.006N) = 81.7^{\circ}[/itex], or the vector points 82 degrees counter-clockwise from west. If you're measuring counter-clockwise from east (as I suppose your teacher is), you get 180 degrees + 82 degrees = 262 degrees. So, it appears you just got your opposite and your adjacent components backwards.

    Please put more effort into writing a clear question next time. Good luck on your test.
    Last edited: Jul 27, 2012
  5. Jul 27, 2012 #4
    Sorry guys! Yes I was very messy. I appreciate all of ur replies. I figured it out at about 2 last night. I'm taking honors physics in 3 weeks and it's very daunting!
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