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Charged cork ball suspended

  1. Oct 3, 2011 #1
    1. The problem statement, all variables and given/known data
    A charged cork ball of mass m is suspended on a light string in the presence of a uniform electric field, as shown in the figure below. When E = (Ai + Bj) N/C, where A and B are positive numbers, the ball is in equilibrium at the angle ?.

    b) Find the tension in the string. (Use any variable or symbol stated above along with the following as necessary: g for the acceleration due to gravity.)

    Here's the picture btw: http://www.webassign.net/pse/p23-55.gif

    2. Relevant equations
    Not really sure.

    3. The attempt at a solution
    I thought I had the answer with (mg)/(-B/A)sin(theta)+cos(theta)...but webassign tells me it's wrong. This is my last problem, and I guess I'm just fried for the day, as it doesn't seem too hard. I have half an hour to submit the answer. Help would really be appreciated.
  2. jcsd
  3. Oct 3, 2011 #2


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    Homework Helper
    Gold Member

    You're going to have to show some more work, I'm afraid. I have no idea where you got the "(mg)/(-B/A)sin(theta)+cos(theta)..."

    That said, I don't think the answer should be a function of θ. I think the answer should be in terms of q, A, B, m, and g. Find the x and y components of T, then use the Pythagorean theorem to find the magnitude. You can find the angle θ (if you need to -- it's not quite clear from the problem statement) by taking the ARCTAN of a relationship between x and y components (and be careful, here; θ is defined with respect to the y-axis, not the x-axis in this problem).

    And yes, you will need a q in there somewhere.
    Last edited: Oct 3, 2011
  4. Oct 3, 2011 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Ah, guess it's too late now.
    Just FYI, it probably would have been helpful to show how you arrived at that answer, in terms of getting other forum members to help out.
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