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Homework Help: Electrostatic calculation

  1. Aug 3, 2015 #1
    1. The problem statement, all variables and given/known data
    A typical inflated balloon has a mass of 4g. Two balloons are rubbed against wool, touched together and then suspended by someone holding the thread at its centre. It is found that the ballooons rest so that trheir centres are 42cm apart and that the thread encloses an angle of 60 degrees. Calculate the charge on each balloon.

    2. Relevant equations
    3. The attempt at a solution
    I first did tan30 *9.8*0.004 (Fgravity) and got 0.226N and then I used F(E)=(k*Q1Q2)/r^2 and since Q1=Q2, I got 2.22x10^-13. But apparently the answer says 6.67x10^-13?
  2. jcsd
  3. Aug 3, 2015 #2
    Do you have a diagram ?
  4. Aug 3, 2015 #3
    The question did not provide one but I think the two ballons are hanging down at rest and their strings shuld form a 60 degree angle from the place where they areheld
  5. Aug 3, 2015 #4


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    Homework Helper

    Why the tangent?
    Best to compare the components perpendicular to the thread.
    That eliminates the tension in the thread.
    Still a good idea to start with a sketch though.
    Few of us can go without such.
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