Finding Tension in Cables: How to Use Moments to Solve for Unknowns

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To find the tension in cables, begin by drawing a free body diagram (FBD) to visualize the forces acting on the system. The center of gravity (CG) for the sign, which has a constant density, will be at its center. Summing the forces in the vertical direction and the moments around a point will help solve for unknown tensions. It's suggested to analyze the system as a whole to simplify the calculations, although breaking it into separate bodies can also be effective. The problem's dimensions may be incorrect, indicating that one of the rods might not be in tension.
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Homework Statement



pysicssign.jpg




Obviously the picture is not to scale

The Attempt at a Solution


I really have no ides where to start I assume you have to use moments to find the tension in each cable but I am really not sure how to go at it.
I think the 20 N force hanging from the sign would only act through the one cable and wouldn't effect the other cable because it is the same distance from the end as the support cable.

Thanks for the help
 
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Start off by drawing your free body diagram. Since the material has a constant density its CG will act at the center of the sign. Next, you find the sum of the forces in the y direction. Also you are correct in saying that you can sum the moments. Just one pointer, try finding the point with most forces acting at it.
 
djeitnstine said:
Start off by drawing your free body diagram. Since the material has a constant density its CG will act at the center of the sign. Next, you find the sum of the forces in the y direction. Also you are correct in saying that you can sum the moments. Just one pointer, try finding the point with most forces acting at it.

I really still don't know where to start can someone go over the steps in doing this question
Thanks
 
Break the figure apart and treat each body as a separate FBD. Show the forces acting on that body and then start writing your equations.
 
Dr.D said:
Break the figure apart and treat each body as a separate FBD. Show the forces acting on that body and then start writing your equations.

Hmm that is too much work, he can do this using 2 equations by treating the whole thing as one object and analyze the tension in the cables alone.
It seems he wants us to do it for him but if he could just separate the body from the wall, then sum the forces in the y direction, and sum the moments (or torques) about either the point where Ta or Tb is acting then he is done.
 
Last edited:
A little bit of discovery is a wonderful learning experience.
 
Haha, i'll place it as a spoiler then ;)
 
The problem statement is incorrect, based on the dimensions shown. One of the rods will not be in tension.
 

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