Free Body Diagram Help for Counterweight Pivot Moment

In summary, the conversation discusses the use of a counterweight connected to an L-shaped link that pivots around a point. The person is trying to determine the moment at the pivot point due to the counterweight, and is unsure which free body diagram (FBD) is correct. They originally think that FBD 1, which assumes the weight of the counterweight acts as a compressive force, is correct but are unsure. The other person in the conversation suggests using FBD 2 and only using the top two arrows to compute the moment about the pivot point. The original person then realizes that FBD 2 makes more sense and asks for clarification on finding the moment in a different scenario. The conversation concludes with the suggestion to use

Which free body diagram is correct

  • Total voters
    3
  • Poll closed .
  • #1
VooDoo
59
0
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
 
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  • #2
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.
 
  • #3
I guess my question to you is why do you think that #1 is correct (which it is not)?
 
  • #4
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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  • #5
Regarding your question in post 4, you can do it either way you prefer, whichever is easiest or readily available.
 

1. Why is a free body diagram important for understanding counterweight pivot moment?

A free body diagram is important for understanding counterweight pivot moment because it provides a visual representation of all the forces acting on an object. This allows you to accurately calculate the magnitude and direction of the forces, which is necessary for determining the counterweight pivot moment.

2. How do I create a free body diagram for a counterweight pivot moment?

To create a free body diagram for a counterweight pivot moment, you first need to identify the object you want to analyze and draw it as a simple shape, such as a dot or a box. Then, draw all the external forces acting on the object, including the weight of the object and the counterweight. Finally, label each force with its magnitude and direction.

3. What is the significance of the counterweight in a free body diagram for counterweight pivot moment?

The counterweight is an essential element in a free body diagram for counterweight pivot moment because it creates an equal and opposite force to the weight of the object being analyzed. This counterforce helps to balance out the weight and allows for a stable pivot point, preventing the object from tipping over.

4. Can a free body diagram help determine the optimal counterweight for a pivot moment?

Yes, a free body diagram can help determine the optimal counterweight for a pivot moment. By accurately representing all the forces acting on the object, you can calculate the moment created by the weight of the object and the counterweight. Adjusting the magnitude and position of the counterweight on the diagram can help you find the optimal balance for the desired pivot moment.

5. Are there any common mistakes to avoid when creating a free body diagram for counterweight pivot moment?

One common mistake to avoid when creating a free body diagram for counterweight pivot moment is not including all the external forces acting on the object. This can result in an inaccurate calculation of the pivot moment and may lead to incorrect conclusions. It is also important to ensure that all forces are correctly labeled with their magnitude and direction.

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