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Homework Help: 3D statics

  1. Jun 20, 2011 #1

    Femme_physics

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    1. The problem statement, all variables and given/known data

    http://img101.imageshack.us/img101/9546/3dstats.jpg [Broken]

    Pole AB is supported at point A by a pin joint, and held at point B by two wires, BC and BD, as described in the drawing. At point B acts on the pole force F which equals 500 [N]. The force F acts on a horizontal plane parallel to Axy, and is slated to an axis parallel to the y axis, and an angle psi that equals 30 degrees.

    Calculate:

    A) Tension on the wires.
    B) That reaction forces at pin joint A

    Measurements in meters.

    3. The attempt at a solution

    http://img202.imageshack.us/img202/720/ohdear1.jpg [Broken]

    http://img838.imageshack.us/img838/9387/ohdear2.jpg [Broken]


    My Az is wrong, my Ay is correct
     
    Last edited by a moderator: May 5, 2017
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  3. Jun 20, 2011 #2

    I like Serena

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    Hi Fp! :smile:

    What do your visual queues tell you about the size of alpha?

    Furthermore, in this Y-Z-view, we will only look at y components and z components of forces.
    What is the Fy component of F?
    Hint: it is not equal to F.
     
  4. Jun 20, 2011 #3

    Femme_physics

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    I don't have visual queues in 3D! Heh. I wished we lived in 2D world calculations would've been easier...

    *deep breath* Oooooooookay.

    So it's actually

    tan-1(2/6) = 18.435

    As far as F.. you're right, I should have Fzy... I don't know it. Argh, this problem is tough.

    Should I try from a different view? This view isn't helping me much.
     
  5. Jun 20, 2011 #4

    I like Serena

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    Now that wasn't so hard was it! :smile:

    Try a top view, just to find Fy.
    Use that in your current view and you should be good.
     
  6. Jun 20, 2011 #5

    Femme_physics

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    Ah, you're right :) As always.

    So I got Ay, and Az, but from some reason I got the wrong BC. I'll scan my attempt when I get back home-- I actually got some work to do today (summer break)^^

    Thanks ILS you're amazing! Cya later :)
     
  7. Jun 22, 2011 #6

    Femme_physics

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    Last edited by a moderator: May 5, 2017
  8. Jun 22, 2011 #7

    I like Serena

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    You calculated BCxz, but BC also has an y component...

    Isn't it fun to wrap your mind around a 3rd dimension! :smile:
     
  9. Jun 22, 2011 #8

    Femme_physics

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    Argh!

    Yes and no! I thought I already got the intuition but looks like I don't still. It'll take some time! But, I'll get there :)

    I wish all the problems we solved so far had been in 3D, the transfer to it is just not that smooth!

    And yet, I still appear to get the wrong BC. The answer in the manual says 476.70 [N]
    This is mine:

    http://img718.imageshack.us/img718/7568/doi0.jpg [Broken]

    http://img27.imageshack.us/img27/355/doih.jpg [Broken]

    http://img853.imageshack.us/img853/7369/doi2.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  10. Jun 22, 2011 #9

    I like Serena

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    I'd be very impressed if you could have done 3D problems straight away! :smile:


    I get 467.70 N.
    If the solution manual says 476.70 N, I think they made a typo (or did you?).

    The fact that you get a slightly different result will be due to rounding in the angles you calculated.
    Actually you should have:
    BCx = 250.000 N
    BCy = 125.000 N
    BCz = 375.000 N

    Did you round the angles to 3 digits (instead of 6 digits)?
     
  11. Jun 23, 2011 #10

    Femme_physics

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    Last edited by a moderator: May 5, 2017
  12. Jun 23, 2011 #11

    I like Serena

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    Last edited by a moderator: May 5, 2017
  13. Jun 23, 2011 #12

    Femme_physics

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    Gooooooooooooooooooooooooooooooooooot it! :) At long lasts!

    Phew, 3D problems are exhausting. I hope the other ones will go smoother. Many thanks master-of-all-things-physics-like :)
     
  14. Jun 23, 2011 #13

    I like Serena

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    Long last? Only 12 posts! :smile:
    You're definitely getting better at this!

    And you're welcome femme-physics :)
     
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