Free Body Diagram Help for Counterweight Pivot Moment

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Discussion Overview

The discussion revolves around calculating the moment at a pivot point caused by a counterweight connected to an L-shaped link. Participants explore different approaches to free body diagrams (FBDs) and their implications for determining the moment, focusing on the effects of link lengths and force components.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents two free body diagrams (FBDs) and questions which one accurately represents the forces acting on the system.
  • Another participant suggests using FBD 2 and only the top two arrows to compute the moment about the pivot point, indicating that the other forces may not be necessary for this calculation.
  • A participant expresses confusion about the validity of FBD 1 and reflects on the role of compression in the link, indicating a shift in understanding towards FBD 2.
  • There is a question about whether to calculate the moment by multiplying the weight by the distance X1 or by separating the weight into components and using the appropriate distances for each component.
  • Another participant states that either method of calculation is acceptable, depending on what is easier for the individual.

Areas of Agreement / Disagreement

Participants do not reach a consensus on which free body diagram is correct, and there are competing views on how to approach the moment calculation. The discussion remains unresolved regarding the best method to use.

Contextual Notes

There are limitations in the assumptions made about the forces acting on the links and the potential effects of link lengths on the moment calculation. The discussion does not resolve these uncertainties.

Which free body diagram is correct

  • Total voters
    3
  • Poll closed .
VooDoo
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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
 
Last edited by a moderator:
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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.
 
I guess my question to you is why do you think that #1 is correct (which it is not)?
 
FredGarvin said:
I guess my question to you is why do you think that #1 is correct (which it is not)?
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!
 

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

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