Determine Zero Force for trusses

[kivine]

Homework Statement

http://img43.imageshack.us/img43/4355/97240590.png

Homework Equations

* If only two members meet in an unloaded joint, both are zero-force members.
* If three members meet in an unloaded joint of which two are in a direct line with one another, then the third (perpendicular) member is a zero-force member.
* If two members meet in a loaded joint and the line of action of the load coincides with one of the members, the other member is a zero-force member.
Question 1. Why is JG not a zero member? is it because of the force acting downwards on JF? If so, what's the general rule about it?

Question 2. If three members meet in an unloaded joint of which two are in a direct line with one another, then the third (perpendicular) member is a zero-force member. What happens if the third member is not perpendicular?

Question 3 Is YZ a zero force member?. With F and without F

 

[nvn]

Hi, kivine. We don't just give out answers to homework questions here. You must attempt to answer your homework questions yourself. That is the third part of the template, which you deleted. After you post a valid attempt, then users might be able to comment on your answers and your current thoughts.

 

[kivine]

Hi, kivine. We don't just give out answers to homework questions here. You must attempt to answer your homework questions yourself. That is the third part of the template, which you deleted. After you post a valid attempt, then users might be able to comment on your answers and your current thoughts.

I guess you misunderstood me. These are some additional queries I've pondered on while doing this question, in which I do have the solution for.

You can see for yourself that I made a poor-ish attempt to illustrate the additional questions I've in the bottom right of the figure. If you want, I can post the solutions, but the solutions have no relation in the questions that I asked.

 

[nvn]

kivine: Thanks for the clarification. No, no need to post the unrelated solutions.

 

[berkeman]

I guess you misunderstood me. These are some additional queries I've pondered on while doing this question, in which I do have the solution for.

You can see for yourself that I made a poor-ish attempt to illustrate the additional questions I've in the bottom right of the figure. If you want, I can post the solutions, but the solutions have no relation in the questions that I asked.

I guess I'm having the same problem seeing your work. Is it a formatting problem? I see the questions at the end of your post, but no work after that...?

 

[nvn]

kivine: Regarding the list of statements with asterisks in post 1, did you copy those statements from a textbook? Or did you (or your teacher) devise those statements yourself?

 

[kivine]

ok.

The statements are found in the wiki and almost any related trusses website.

There's no work after the questions because the questions are in fact questions that came from me and no one else. And also because they are simply direct questions and if you needed *workings* they can be found in the statement.How else do you define workings then if the question is not a mathematical based question!? My understanding are based on the top 3 statements. so if workings = understanding, then you got it

If so here's what you guys need
: http://img32.imageshack.us/img32/9525/62708875.png

 

[nvn]

kivine: Do not believe everything you read on wikipedia. Remember, wikipedia can sometimes be written by unknowledgeable amateurs. The wikipedia page you are quoting is a prime example, because it currently contains multiple misstatements, which would tend to confuse any reader. Here are the correct facts.

(1) If only two noncollinear members meet in an unloaded joint, both are zero-force members.
(2) If only three members meet in an unloaded joint of which two are collinear, then the third (noncollinear) member is a zero-force member.
(3) If only two noncollinear members meet in a loaded joint, and the line of action of the load is collinear with one of the members, the other member is a zero-force member.

Moving on to your questions.

(Q1) "Why is GJ not a zero-force member? Is it because of the force acting downwards on FJ? If so, what's the general rule about it?"

Member GJ is not a zero-force member because member FJ applies a component of force (to joint J) perpendicular to members CJ and EJ. Therefore, the only member that can resist this force component is member GJ.

(Q2) "[Regarding rule 2,] what happens if the third member is not perpendicular?"

Nothing. The corrected rule is written above.

(Q3) "Is YZ a zero-force member? With F and without F?"

Just taking a quick glance, it appears your structure is a mechanism (unstable). (But I hope PhanthomJay or someone like that will give a second opinion, if I am misinterpreting.) Therefore, the question is somewhat superfluous, because the structure is unstable; but I will try to answer it anyway, for the purpose of this question. Without F, member YZ would tend to be a zero-force member, per rule 2, above. With F, member YZ would tend to not be a zero-force member. But again, the structure is unstable, so there is not an exact answer to the question.

 

[PhanthomJay]

I completely agree with nvn's above response.

 

[kivine]

thanks a lot for the help :)