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Force pulling a pendulum

  1. Oct 31, 2013 #1
    1. The problem statement, all variables and given/known data

    A 2.0 kg pendulum ball is pulled 30° from the vertical by a horizontal force Fpull. What is the magnitude of Fpull?

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    I'm rather lost with this one - I'm not even sure how to classify it. Any help at all - even a point in the right direction - would be greatly appreciated.
     
  2. jcsd
  3. Oct 31, 2013 #2
    Unless the problem says otherwise, assume Fpull is at a 90° angle from the vertical.

    Force diagram. You'll have three forces: tension, Fpull, and the force of gravity. The tension will have two components.
     
  4. Oct 31, 2013 #3
    sorry, misread the post. it said horizontal. So it is definitely a right angle from the vertical
     
  5. Oct 31, 2013 #4
    Yes -- diagram it.
     
  6. Nov 1, 2013 #5
    Hi all, thanks for your input. Is it right to assume that the force of gravity = the y component of the tensional force, and that the force applied to the ball = the x component of the tensional force? If so, how can I calculate Tx without knowing the magnitude of T? (If my assumptions are correct, I can calculate Ty using Fg, but I don't know how that value could help me.)
     
  7. Nov 1, 2013 #6

    haruspex

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    Yes.
    If T is the total tension and the angle is 30o, what in terms of T are Tx and Ty?
     
  8. Nov 1, 2013 #7
    If the ball is 30o from the vertical to the right, then T is oriented 130o from the positive direction of the x-axis. Then Tx =Tcos130, and Ty=Tsin130?
     
  9. Nov 1, 2013 #8

    haruspex

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    No, not 130. How did you calculate that?
     
  10. Nov 1, 2013 #9
    Oops, I meant 120. I've attached a picture (.jpg) showing how I calculated the angle.

    Thanks for your responses so far!
     

    Attached Files:

  11. Nov 1, 2013 #10

    haruspex

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    Yes, that's better.
     
  12. Nov 1, 2013 #11
    I've got the answer :)

    For anyone who might come across this post in the future:

    Ty=Fg=mg=2*9.8=19.6=Tsin120
    So T=22.63
    |Fpull|=|Tx|=|Tcos120|=|-11.3|=11.3
     
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