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Homework Help: Finding the E-field of a mass hanging from a string

  1. Jul 7, 2011 #1
    9. [1pt]

    As shown in the figure above, a ball of mass 0.480 g and positive charge q =32.7microC is suspended on a string of negligible mass in a uniform electric field. We observe that the ball hangs at an angle of theta=15.0o from the vertical. What is the magnitude of the electric field?

    Fx= Fe - Tsin(theta)= 0
    Fy= Tcos(theta) = 0

    Tsin(theta)/Tcos(theta) = Fe\mg

    Than E= Fe/q

    So Tan(theta)mg = Fe/q = E

    I got (Tan(15)*.480*9.8)/32.7e-6= 3.9 e 4
    Where did I go wrong?
  2. jcsd
  3. Jul 7, 2011 #2


    User Avatar

    Staff: Mentor

    Note the units in which the mass was specified.

    EDIT: Note also that since this is a homework problem, it should have been posted in one of the homework forums, in particular the Introductory Physics Forum.

    EDIT by mentor: this thread has been moved to the appropriate location.
    Last edited by a moderator: Jul 8, 2011
  4. Jul 7, 2011 #3
    Its right! Thank you for responding so fast!
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