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Why is component reverse in Conical Pendulum?

  1. Nov 20, 2015 #1
    Hello. In class today, we studied conical pendulum but I was confused at the part about its components. In all the diagrams ( http://dev.physicslab.org/img/90c0fb7a-ffb4-4573-b389-b50a559732c8.gif ) if shows The x-component as being sine, but I always thought x-comp was cos?? ( http://faculty.ccbcmd.edu/~jzelinsk/phys_101/chapter3/chapter3.htg/img2.gif [Broken] )



    Also for equations I made them

    T cos(θ) = mv2/r
    T sin(θ) = mg

    But teacher say it is wrong and it should be other way but I dont understand, please help!
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Nov 20, 2015 #2

    TSny

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    Often the direction of a vector is specified by the angle the vector makes with respect to the x axis, as in your second link. What about θ in your first link?
     
  4. Nov 20, 2015 #3
    But even it was at a different spot, wouldn't the horizontal always be cos?
     
  5. Nov 20, 2015 #4

    TSny

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    Look at the figure below. How would you use trig to find the lengths of the green and red sides of the triangle?
     

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  6. Nov 20, 2015 #5
    Green would be sin and red would be cos. so

    T cos(θ) = mv2/r
    T sin(θ) = mg
     
  7. Nov 20, 2015 #6

    TSny

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    Yes, that's right.
    Which side of the triangle "points toward the center" of the circular motion of the pendulum bob?
     
  8. Nov 20, 2015 #7
    Red line I believe
     
  9. Nov 20, 2015 #8

    TSny

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    Where is the center of the circular motion in the picture in your first link? Is it to the left of the mass, to the right of the mass, above the mass, or below the mass?
     
  10. Nov 20, 2015 #9
    I think its to the left
     
  11. Nov 20, 2015 #10

    TSny

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    Yes.

    So, according the the figure below, which component of the tension points toward the center of the circular motion? The green component or the red component?
     

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  12. Nov 20, 2015 #11
    Green comp
     
  13. Nov 20, 2015 #12

    TSny

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    Yes. So, which component should be set equal to mv2/r?
     
  14. Nov 20, 2015 #13
    sine!!:)
     
  15. Nov 20, 2015 #14

    TSny

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    Good.
     
  16. Nov 20, 2015 #15
    Just a quick question,so the x-comp can always change? So it is not always cos or sin but it depends on where the center of motion is right?
     
  17. Nov 20, 2015 #16

    TSny

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    The x component is not always obtained by using cos(θ). It depends on whether or not θ is measured from the x axis. Whenever you are in doubt, construct a right triangle and use trig to get the x and y components.
     
  18. Nov 20, 2015 #17
    Oh ok got it thanks :)
     
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