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Homework Help: Ball Balanced by Forces

  1. Jan 20, 2015 #1
    1. The problem statement, all variables and given/known data
    A single charge q1 = 1.10E-7 C is fixed at the base of a plane that makes an angle theta with the horizontal direction. A small ball of mass m = 1.75 g and a charge q2 = 3.30E-8 C is placed into a smooth frictionless groove in the plane that extends directly to the fixed charge (image attached.) It is allowed to move up and down until it finds a stable position d = 11.0 cm from the fixed charge. What is the value of theta (t)?
    2. Relevant equations
    Fe = kq1q2/d^2
    Fg = mg

    3. The attempt at a solution
    Fe = (8.99E9)(1.1E-7)(3.3E-8)/(0.11^2) = 0.002697 N [up to the right]
    Fg = (.00175)(9.8) = 0.01715 N [down]

    (Fe)(sin(t)) = mg
    (0.002697)(sin(t)) = 0.01715
    sin(t) = 6.358917316
    Since sin(t) has to be between -1 and 1 it is impossible to find t.

    Any help detecting my problem would be greatly appreciated.


    Attached Files:

  2. jcsd
  3. Jan 20, 2015 #2
    I think you forgot about the normal force.

    Try to set up the vector decomposition differently.
  4. Jan 21, 2015 #3


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    That's wrong. Which direction are you resolving in to get that?
  5. Jan 21, 2015 #4
    So my new equation would be
    mg = Fe(sin(t)) + N.
    I'm confused as to where to go from here.
    What did you mean by "set up the vector decomposition differently."
  6. Jan 21, 2015 #5
    I'm resolving along the y-axis (vertical). If you look at the photo, I think it is right.
  7. Jan 21, 2015 #6
    You found the vertical component of Fe.

    If you take the plane to be the x-axis, you can find the horizontal component of mg.
  8. Jan 21, 2015 #7


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    No, as you replied to showzen, it wasn't right because you had left out the normal force.
    The simpler path, though, is to resolve parallel to the plane so that you don't need to involve the normal force.
  9. Jan 21, 2015 #8
    Sorry, when you said "That's wrong," I thought you meant that I got the direction, as in I used the wrong trig function.
  10. Jan 21, 2015 #9
    Thanks for your help. Changing my coordinate system makes the whole problem much easier.
  11. Jan 21, 2015 #10


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    I couldn't tell which - either you'd resolved vertically and omitted the normal force, or you'd resolved down the plane and put the sine factor on the wrong side. I've seen the second done more often than the first.
  12. Jan 21, 2015 #11
    Thanks again for all your help! This forum is very helpful to fully understand where I am going wrong.
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