# Three forces, one graph, one crazy problem

1. Sep 18, 2008

### Phoenixtears

1. The problem statement, all variables and given/known data
Three forces are applied to an object, as indicated in the drawing. Force 1 has a magnitude of 33.0 newtons (33.0 N) and is directed 30.0° to the left of the +y axis. Force 2 has a magnitude of 21.0 N and points along the +x axis. What must be the magnitude and direction (specified by the angle in the drawing) of the third force 3 such that the vector sum of the three forces is 0 N?
(Image attached)
magnitude N
theta °

2. Relevant equations

F=ma

3. The attempt at a solution

I have made the entire table involving the magnitudes, directions, Fx, and Fy of the three forces plus the totals overall. Earlier in class we worked a little bit on vector sums, but now I realize that I don't know whether or not the vector sum involves the Fx or Fy. I've attempted to add the bits together (I figured out the Fx and Fy's by using cosine and sine) for instance, for F1 the Fy force is 28.6 and the F2 Fy force is 0. However, the answer is not 28.6. What am I doing incorrectly?

Thanks,
Phoenix

#### Attached Files:

• ###### Force Graph.gif
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2. Sep 18, 2008

### rl.bhat

For equilibrium F1y = F3y and F1x + F3x = F2
From these two equations you can find F3 and the angle.