How Do You Calculate Tension in Different Cord Configurations?

[charlie_luna]

Homework Statement

An 80 N traffic light is supported at the midpoint of a line between two poles that are 30 m apart. If the light sags a vertical distance of 1 m at the midpoint, what is the tension in the supporting line?

A 20 lb picture is hung from a nail so that the supporting cords make an angle of 60. what is the tension of each cord segment?

Here is the actual homework problem i have to solve: An 80 N picture is hung from a nail as indicated. Assume the zeros in the 80 N weight are significant. The angle between the cords in 78 degrees. What is the tension in the each cord?

Homework Equations

I am lost here.

The Attempt at a Solution

And lost here.

There are 2 examples in my instructor's lecture notes that he sends to us by mail that has a street light and another picture hanging from 2 cords. in the street light problem, the tensions are all positive:

Tax = TaCosdegree
Tay = TaSindegree
Tbx = TbCosdegree
Tby = TbSindegree

and so on. But in the picture problem, some of the equations have a negative out front. such as:

Tbx = TbCos60
Tby = TbSin60
Tax = -TaCos60
Tay = TaSin60.

why is that? why would some problems be all positive and some get negatives in others? I have realized that i am suppose to set EFx = 0. where the E is the sigma symbol which i believe stands for equilibrium or net. and i am suppose to set EFy = 0.

can someone please help me understand this? my instructor has been out sick since last monday with swine flu and he has not returned to work yet.

 

[Redbelly98]

Welcome to PF.

It's best to start by drawing a free-body diagram. For these problems, there should be 3 forces acting on the object.