Simple degree help (tan-1 fy/fx)

[BunDa4Th]

Homework Statement

Three point charges are located at the corners of an equilateral triangle as shown in Figure P15.13 (q = 3.00 µC, L = 0.400 m). Calculate the net electric force on the 7.00 µC charge.

° (counterclockwise from the +x axis)

http://www.webassign.net/pse/p23-07alt.gif

The Attempt at a Solution

F_12 = 8.9875 x 10^9 (3e-6)(7e-6)/.4e2 = 1.18 N

F_13 = 8.9875e9 (7e-6)(4e-6)/.4e2 = 1.57 N

then

F_12x = 1.18cos60 = .59 N
F_12y = 1.18sin60 = 1.0219 N
F_13x = 1.57cos60 = .785 N
F_12y = -1.57sin60 = -1.3697 N

then i added them up

Fx = .59 + .785 = 1.375 N
Fy = -1.3597 + 1.0219 = - .3378 N

I know that to get the degree its arctan Fy/Fx but i get the incorrect answer. Is there something wrong in the equation or I am i using the wrong number?

I understand how to do the first part which is 1.4 N of electric force on 7uC charge.

Thanks for any help.

 

[hage567]

What angle are you getting??

 

[BunDa4Th]

i get -13.80*

if i add 180 to that i get 166.2*

both are incorrect.

 

[hage567]

Wouldn't you want to add 360 to it? That would give you the angle from the x-axis going counterclockwise instead of clockwise. Just a guess.

 

[BunDa4Th]

You are correct. it is adding 360.

I thought it was 180 because i was thinking of a triangle which is 180*.

Thanks again.