Help understanding Truss answer

[NoobeAtPhysics]

Homework Statement

The truss is suspended by two pin joints. Each segment is 3 m wide and 4 m high. The applied force is F = 1 N. What is the force in member 2?

Homework Equations

Sum of force = 0

The Attempt at a Solution

Using method of sections (to get force at 3), so taking the right portion

I am trying to understand the sum of the force in the y direction of the section.

sum of force y of portion =0 =-1 - J_3 cos(180-arctan(3/4))

My question, do we use 180-angle when looking at the angle,

also is it negative J_3, since the J_3 is pointing downwards?


I already know that the answer is 1N, I just don't understand this part.


Thank you.

 

[NoobeAtPhysics]

Any ideas?

 

[PhanthomJay]

I think you are looking for force in 3 by cutting right hand section thru 12,3, and 9? Forget about signs plus or minus from trig. You know that F acts down, so the vert force in 3 must act up, pushing in towards the member. Pushing forces are compression forces, usually designate as negative. Tension forces are positive. Use the properties of a 3-4-5 right triangle to get the compressive force in member 3.