Need help finding tension in cables

[Nanart]

Homework Statement

Homework Equations

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 on cable and wouldn't effect the other cable because it is the same distance from the end as the support cable.

Homework Statement

Thanks for the help I have been trying to figure this out for a while

 

[PhanthomJay]

Are you sure of those dimensions? That sign doesn't look stable.

 

[Nanart]

Yes those are the dimensions...its just really out of wack for the question obviously no one would build a sign like that

 

[PhanthomJay]

But the scale is way off; the first support rod is way off to the right, almost right next to the other support rod. You don't show it that way. And both rods would not be in tension, with the dimensions shown.

 

[Nanart]

But the scale is way off; the first support rod is way off to the right, almost right next to the other support rod. You don't show it that way. And both rods would not be in tension, with the dimensions shown.

Sorry i didnt really mention that...It is out of a textbook and it is not scale..dont know why so just ignore the scale

 

[PhanthomJay]

Something then is amiss, because the question asks for the tension in each rod. The second rod at the right will not be in tension. I think the problem means to say
a = 0.1 m and b = 1.0 m. You are correct in that the 20 N weight just gets carried by the right rod only. But the force value in that rod from the sign weight will far outnumber the 20 N force in that rod, and put it into compression. Try summing moments about the first rod, applying the sign's weight at the center of the sign.

 

[Nanart]

Something then is amiss, because the question asks for the tension in each rod. The second rod at the right will not be in tension. I think the problem means to say
a = 0.1 m and b = 1.0 m. You are correct in that the 20 N weight just gets carried by the right rod only. But the force value in that rod from the sign weight will far outnumber the 20 N force in that rod, and put it into compression. Try summing moments about the first rod, applying the sign's weight at the center of the sign.

I know the question is right...If anyone can explain how to do it I would really appreciate it

 

[PhanthomJay]

Unless I'm misreading your a and b dimensions, the question is improperly worded, or the dimensions given are wrong.

 

[Nanart]

Unless I'm misreading your a and b dimensions, the question is improperly worded, or the dimensions given are wrong.

This question is right out of a physics textbook

 

[PhanthomJay]

And Physics books once in awhile make errors; their authors are human, you know.

 

[Nanart]

And Physics books once in awhile make errors; their authors are human, you know.

I had my teacher explain this question to me and I don't know why phantomJay thinks the second rod isn't under tension when it clearly is.. to find the tension you just have to select a point and find the moments around that point which will give the tension on on rod, you than subtract that from the total weight since the tension will equal the total weight and that gives you the tension on the second rod.

So just to clarify it was not a typo

 

[nvn]

Nanart: The right-hand, upper support rod is in compression. Perhaps if you show your work, we could locate your mistake.

 

[mathmate]

Nanart,

If you are a practising engineer or physicist, you will find that it does help to sketch your problem reasonably to scale. Many mistakes (including those from textbooks) are due to improperly drawn and hence misleading sketches.

You can sit there and insist that the question is straight from the textbook, but it's not going to help you. Textbooks make mistakes, we all know it.

I suggest you make a sketch reasonably to scale, re-do your moment calculations and post the results, as nvn suggested. When you are convinced, show your teacher the sketch and the calculations, then you'd be getting somewhere.