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Free Body Diagram Help

Which free body diagram is correct

Poll closed Feb 8, 2010.
  1. FBD 1

    0 vote(s)
  2. FBD 2

  1. Jan 9, 2010 #1
    Hey guys,

    I have a counterweight that connects to an L-shaped link that pivots about a point.

    I need to work out the moment at the pin due to the counterweight, but I am stuck on the basics.

    The first free body diagram I drew assumes that the weight of the counterweight acts entirely as a compressive force along the first link. The second assumes that this force is broken into x and y components.

    I believe that the length of link 2 has an effect on the moment at the pivot point. I.e. as the length of link 2 is increased the moment at the pivot point will change, is this correct?

    I am stuck figuring out which FBD is correct.

    Any help is appreciated!

    Edit - Having trouble uploading the image, so I used imageshack

    http://img511.imageshack.us/img511/8489/fbdp.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 9, 2010 #2


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    Use FBD 2. And use only the top two arrows on FBD 2 to compute the summation of moment about the pivot point. You do not need the other four arrows on FBD 2, unless there is something else you need to do besides computing the moment about the pivot point.
  4. Jan 9, 2010 #3


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    I guess my question to you is why do you think that #1 is correct (which it is not)?
  5. Jan 10, 2010 #4
    Thanks very much for the replies. To be honest, I originally had FBD 2 down, but then the thought of the link being in compression threw me off for some reason. But the logic behind FBD 2 kind of makes sense out.

    Just for the sake of learning, I have drawn up an arbitrary arrangement and attached it. If I were to find the moment that this weight creates at the pivot point, would I be simply multiplying the weight by the distance X1 or would I be separating the weight into components parallel and perpendicular to the links and multiplying them by the appropriate distance?

    Thankyou very much for the help, it is much appreciated!

    Attached Files:

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    Last edited: Jan 10, 2010
  6. Jan 10, 2010 #5


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    Regarding your question in post 4, you can do it either way you prefer, whichever is easiest or readily available.
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