Tension and Compression in Truss Joints

[Hyperfluxe]

Statics - Forces in a Truss

Homework Statement

http://i.imgur.com/NbZlc.png

Homework Equations

Equations of equilibrium (sum of the forces in all directions = 0, sum of the moments about an point = 0)

The Attempt at a Solution

My attempt: http://i.imgur.com/qlLCY.jpg
Hopefully you can read it. What I did is that I did the FBD of the entire system neglecting the internal forces, and I found the reaction forces. Then I did a FBD for joint A alone and solved for the two forces. Me and my friends are getting different answers so I just want to make sure. What I'm unsure about is my signs and the whole tension vs. compression thing. I don't understand it at all. Let me know where I went wrong (if I went wrong). Thanks!

EDIT: I'm not really sure why this was moved to the engineering section, this problem is from my Statics course which is an introductory physics course (right?).

 

[SammyS]

Homework Statement

http://i.imgur.com/NbZlc.png

Homework Equations

Equations of equilibrium (sum of the forces in all directions = 0, sum of the moments about an point = 0)

The Attempt at a Solution

My attempt: http://i.imgur.com/qlLCY.jpg
Hopefully you can read it. What I did is that I did the FBD of the entire system neglecting the internal forces, and I found the reaction forces. Then I did a FBD for joint A alone and solved for the two forces. Me and my friends are getting different answers so I just want to make sure. What I'm unsure about is my signs and the whole tension vs. compression thing. I don't understand it at all. Let me know where I went wrong (if I went wrong). Thanks!

EDIT: I'm not really sure why this was moved to the engineering section, this problem is from my Statics course which is an introductory physics course (right?).

Your solution looks fine to me. I didn't check the numbers, but they appear to be in the right ballpark.

From my experience, a Statics course is generally Engineering, even if it's taught by the physics department. (No, I wasn't the one who moved it.)

 

[Hyperfluxe]

Thank you :)
A lot of people in my class seem to be getting 23.7N for the axial force in AE, and they found that the joint is under tension. I don't see anything wrong with my solution so I'm keeping it.

I'm confused about one thing: how exactly do you know if a member is under compression or tension? For example, for my free-body diagram at joint A, both axial forces point towards the joint, so I call that compression. Is that how it works? Also, when you get a negative answer for a force or reaction force, do you carry that negative sign in the rest of your equations? I know that you have to change the direction on the FBD after.

 

[Ziv7]

In calculations compression units are negative and tension are positive

if you have 2 forces going away from the beam you would draw arrows inside the beams going in [ -> <- ] (opposite to reaction) - that beam would be in tension

this is how i was taught to do it, hope it helps.

 

[SammyS]

Thank you :)
A lot of people in my class seem to be getting 23.7N for the axial force in AE, and they found that the joint is under tension. I don't see anything wrong with my solution so I'm keeping it.

They would get 23.7N by adding 17 + 17cos(60°) . But those forces are in opposite direction, so you're solution is correct.

I'm confused about one thing: how exactly do you know if a member is under compression or tension? For example, for my free-body diagram at joint A, both axial forces point towards the joint, so I call that compression. Is that how it works? Also, when you get a negative answer for a force or reaction force, do you carry that negative sign in the rest of your equations? I know that you have to change the direction on the FBD after.

 

[PhanthomJay]

Always, when a force of a member on a joint points toward the joint, it is in compression; if the force in a member on a joint points away from the joint, it is in tension. As Sammy S has noted, your answer is correct. And yes, if you get a negative answer, you assumed the wrong direction, and must reverse the direction on your FBD.

 

[Hyperfluxe]

Thanks so much guys, I appreciate it =)