# Method of joints, force in each member

by togo
Tags: force, joints, member, method
 P: 106 1. The problem statement, all variables and given/known data Using method of joints, determine the force in each member of the truss. State whether each member is in compression or tension. 2. Relevant equations joint method, I'm not sure how to describe it. 3. The attempt at a solution The answer is of course Fab = 261.8 lb T Fbc = 78.9 lb C Fac = 39.4 lb T
 P: 106 did you guys actually remove his post? It's very difficult to get anyone to help you here, why would you ever do that. Its the only resource I know of.
 Emeritus Sci Advisor HW Helper PF Gold P: 7,271 Do realize how hard it is to figure out what you did by looking at that image? It looks like there is some work relating to other problems. I'll try to take a closer look soon.
P: 106

## Method of joints, force in each member

alright so I checked out the example again and can't quite see whats wrong with my method (force triangles). Isn't it possible to break down that angled force into horizontal and vertical component, two separate forces, then use a force triangle for each force and add up the resultants somehow?
P: 106
 Quote by SammyS Do realize how hard it is to figure out what you did by looking at that image? It looks like there is some work relating to other problems. I'll try to take a closer look soon.
I'm not asking you to decipher what I did. I pretty much just posted it out of desperation.

and yes there is calculus on the top of the page
 P: 106 This is the example question I base my method off. To find Fab & Fad: 200lb/4 = Fab/3 = Fad/5 (although the previously erased solution is making sense to me, I'm not sure if a question asks you to solve method of joints, that you are allowed to use moments? http://img291.imagevenue.com/img.php..._122_404lo.jpg http://img158.imagevenue.com/img.php..._122_257lo.jpg http://img255.imagevenue.com/img.php..._122_240lo.jpg http://img240.imagevenue.com/img.php..._122_538lo.jpg)
 P: 3 I'm the guy who gave the other solution. It had later occurred to me about using moments. I realize that it would have been easier to do a force triangle at joint B. Unfortunately, I can't quite follow your photographed page I think that it's still method of joints so long as you use joint method to find each member. I just used moments to find one of the reactions first. If you have the time to post a better picture of how you tried to solve it then I'm sure that someone will 'lawfully' tell you what you did wrong (or I will try to).
 P: 106 again thanks for your help, the question is not typical of the kind coming out of this chapter.. (ch6 of mechanics for engineers, statics, 5th ed) as the forces in the other questions are all perpendicular. So I'm moving on to them.
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P: 7,271
 Quote by togo 1. The problem statement, all variables and given/known data Using method of joints, determine the force in each member of the truss. State whether each member is in compression or tension. 3. The attempt at a solution http://i41.tinypic.com/2w6rkax.jpg The answer is of course Fab = 261.8 lb T Fbc = 78.9 lb C Fac = 39.4 lb T
I get that Fcv=68.297 lb (up).

However, that is based on moments, which I believe is what you did. That's not the method of joints.

By the method of joints, joint B is in equilibrium (as are all the joints), so
ƩFx=0 → (FAC)cos(50°) - (FBC)cos(60°)+240=0

ƩFy=0 → (FAC)sin(50°) + (FBC)sin(60°)+100=0

(I chose the signs to assume each beam was under compression.)
 P: 106 one more question though, is there a way to know which members are in tension or compression?
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P: 7,271
 Quote by togo one more question though, is there a way to know which members are in tension or compression?
Yes, of course.

Use a sign convention. In my suggestion, I assumed compression was positive.

BTW: You should be able to check your solution by comparing what you got using moments.
P: 106
 Quote by SammyS I get that Fcv=68.297 lb (up). However, that is based on moments, which I believe is what you did. That's not the method of joints. By the method of joints, joint B is in equilibrium (as are all the joints), soƩFx=0 → (FAC)cos(50°) - (FBC)cos(60°)+240=0 ƩFy=0 → (FAC)sin(50°) + (FBC)sin(60°)+100=0 (I chose the signs to assume each beam was under compression.)
You know I'm not quite seeing this, would it be possible for you to do a free body diagram? thanks

(shouldn't Fac in your equation actually be Fba?)

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