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FBD Problems

  1. Oct 23, 2005 #1
    Okay, started these problems with just the FBD drawing parts, but need some more clues as to how to go about starting them, i.e., just what equations/order is used to set them up, etc.:

    1) Supposed to write the 6 equations of equilibrim, so, for example, is this what I need to do:
    Max+May+Maz+Fax+Fay+Faz+T+L+Mwing = 0 ?

    2) Similar procedure as #1?

    3) This looks a lot like the moment problems I just got done doing--am I looking for forces, moments, or both at A?

    4) Again, I'm guessing a similar setup as before (if I started that right?)?

    Also, for #1,2,4 should the weight "W" be included in the FBD?
    Last edited: Oct 23, 2005
  2. jcsd
  3. Oct 23, 2005 #2


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    Your FBD is good. Not the equations, though. You can't just put all the terms together and call that a meaningful equation.

    You have to learn how to write out the scalar equations that result from Newton's Second Law (and for balanced torques). This is explained in your text. Read it first.

    Also, you will find tutorials at the top of this forum - a couple of which teach you how to deal with Newtonian mechanics problems.

    PS : This is introductory physics, not advanced physics - just so you know where to post next time.
  4. Oct 23, 2005 #3

    Actually, this is mid-semester Statics stuff, so I just thought this would be a good place to post it (even though it is, really, just FBD's).

    "Not the equations, though. You can't just put all the terms together and call that a meaningful equation."

    Yeah, I kinda just saw that--that's more of the "theory" behind the problem to start than anything really meaningful I guess.

    So, do I need the W term or not in the FBD's?
  5. Oct 23, 2005 #4


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    Are you supposed to find the reactions? If so make a resultant-couple system about O to find the moments (decompose the moment vector into components in x,y,z axis). For the airplane problem.
  6. Oct 23, 2005 #5
    Yeah, and for the others too:
    #1, supposed to find "the components of reaction where the wing is fixed to the fuselage at A".
    #2, find "the components of reaction at the ball and socket joint A and then tension in the supporting cables BC and BD."
    #3, find "the reactions at the fixed wall A."

    but then
    #4 says "if the hoist is in equilibrium, what forces act on the shaft at A? What is the max. mass m in kg that can be lifted?"

    For the airplane one, what is the moment vector? r x L?
  7. Oct 23, 2005 #6


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    the resultant moment of the forces, now simply descompose in x,y,z axis to find the values of the reaction moment components.

    [tex] \vec{M}_{R} = \vec{r}_{1} \times \vec{L} + \vec{r}_{2} \times \vec{T} + \vec{r}_{3} \times m \vec{g} [/tex]
    Last edited: Oct 23, 2005
  8. Oct 23, 2005 #7
    Oh, okay, that makes sense.
    Last edited: Oct 23, 2005
  9. Oct 23, 2005 #8
    Okay, ended up with this for #1:
    MR=(572,100i - 20,000j - 64,000k)Nm

    It had said to "write equations of equilibrium--6 eq's and 6 unknowns". Do I really even have to do that, since I basically just found the components of reaction at A?
  10. Oct 23, 2005 #9


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    You found the components of the Moment vector reaction at A, but there is also a Force vector Reaction at A!
  11. Oct 23, 2005 #10

    So, that would just be each F multiplied by its unit vector?
    Last edited: Oct 23, 2005
  12. Oct 23, 2005 #11


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    Yes, descompose the forces in unit vectors and work out the equations.
  13. Oct 23, 2005 #12
    So, is this right for force L (just one of them):
    LA=45,000*(-7j/7)= -45,000j??
    Last edited: Oct 23, 2005
  14. Oct 23, 2005 #13
    Does this look right? Then I found TA*r and WA*u the same way.

    To get:
    TA=-7,120j + 3,600k
    then FA=(0i - 34,540j + 3,600k)N?

    Does there being no x(i) value seem wrong? Does to me.
    Last edited: Oct 23, 2005
  15. Oct 23, 2005 #14
    Something with the final force components is confusing me...
  16. Oct 23, 2005 #15
    Did I calculate the reactive forces for #1 right or not?

    Moving on, would #2 be the same thing as this last part of 1, finding the reactive forces about the ball and socket joint? Then how do I get the tension forces?
  17. Oct 23, 2005 #16
    Where'd everyone go? Dinner?:tongue2:
  18. Oct 23, 2005 #17
    Anyone? Did I calculate the Force components at A correctly or not?
  19. Oct 23, 2005 #18


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    Your descomposition in unit vectors of the Force L is does not "works" with your defined sign convention because of your set axis.
  20. Oct 23, 2005 #19
    Okay, so? I'm not feeling too well today, sorry--but what exactly do I change then? Seems VERY simple, but something isn't clicking.:frown:
  21. Oct 23, 2005 #20


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    Well +Z should be the positive so:

    [tex] \vec{L} = 45000 \vec{k} [/tex]
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