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3D statics - is there a simpler, shorter way to solve problems?

  1. Jun 19, 2011 #1

    Femme_physics

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  2. jcsd
  3. Jun 19, 2011 #2

    I like Serena

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    I take it you mean these drawings?
    http://imageshack.us/photo/my-images/41/3dattempt.jpg


    Well, there is a method using 3D vectors and outer vector products, but I think you did not meet those friends yet.
    And even with them, you're actually doing the very same thing! :wink:

    So the answer, in my humble opinion, is: no, there is no shortcut.
    You have to make all 3 drawings.

    On the other hand, your calculations could have been a bit shorter.
     
  4. Jun 19, 2011 #3

    Femme_physics

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    Yes I meant those drawing.


    "On the other hand, your calculations could have been a bit shorter.
    "

    How?
     
  5. Jun 19, 2011 #4

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    You kept using trigonometry, to calculate force components from lengths.
    This is not necessary.

    It's simplest if you always use the force components, like Tx, Ty, Tz.
    And that you do not keep calculating from a projection of T with an angle.
     
  6. Jun 19, 2011 #5

    Femme_physics

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    True, but that can be said of 2D's as well. This is not the part that really delays me I just wanna write things in details. :)

    What I saw my lecturer do in class quite baffled me. He did sum of all moments to the AXIS! I didn't think that was possible..? So I was fairly confused in class.
     
  7. Jun 19, 2011 #6

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    Let me see if I can explain.

    A "moment" is a measure how strongly a body will rotate.
    In 2D a body will rotate around a point.
    In 3D a body will rotate around an axis.
    When you make a 2D drawing from a 3D problem, the 3D axis turns into a 2D point.

    If you have a pole and look at it, it looks like a line doesn't it?
    Now if you keep the pole straight away from your eye, it starts to look like a point, does it not? :smile:

    If you want to calculate a moment in 3D in respect to a specific axis, you can make a 2D projection along the axis and do it as you're used to.
    Or you can try and calculate the various distances to this axis in 3D.
     
  8. Jun 19, 2011 #7

    Femme_physics

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    I see.

    [/quote]
    Or you can try and calculate the various distances to this axis in 3D.
    [/quote]

    I don't think it's all that difficult to do, but calculating moment to an axis gives me more unknowns, no?

    I'll show what I mean in a 3D statics problem I'll post.
     
  9. Jun 19, 2011 #8
    I'm sorry to break it to you that much (even most) engineering calculation is copious tedious repetition of some simple formula.
    Of course, since it is simple, it lends itself well to automatic (computer) calculation and there are many computer programs available to do just that.

    But you still need to understand what the computers are doing otherwise garbage in = garbage out.

    I once had a program (Windows 3.1) called Design View.

    It was a marvelous program that allowed you to draw vectors and automatically prepared a spreadsheet for you and performed the calculations straight off your drawing.
    If you then amended the spread sheet it would amend the drawing or if you amended the drawing it would recalculate the spreadsheet.

    Marvelous, but I have never managed to get it to run successfully on later versions of Windows, although I still keep the original disks.
     
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