Alright.
The Question states:

The drawing shows an equilateral triangle, each side of which has a length of 2.00 cm. Point charges are fixed at each corner, as shown. The 400 mC charge experiences a net force due to the charge q A and qB. This net force points vertically downward in the drawing and has a magnitude of 405 N. Determine the magnitude and algebraic sighs of the charges qA and qB.

This is what I did:

(1) I know that centimeters are supposed to be converted to meters
2.00*10-6
(2) Two vectors A & B are added together to produce a third vector C = A + B .
(3) When the triangle formed is not a right triangle:
the magnitude of the vector sum can be found by using the Law of Cosines or the Law of Sines

meaning:

4.00*10-6
-----------------------
1

so now I am lost
HELP!!

LowlyPion
Homework Helper
(1) I know that centimeters are supposed to be converted to meters
2.00*10-6

(2) Two vectors A & B are added together to produce a third vector C = A + B .
(3) When the triangle formed is not a right triangle:
the magnitude of the vector sum can be found by using the Law of Cosines or the Law of Sines

meaning:

4.00*10-6
-----------------------
1

so now I am lost
HELP!!
Welcome to PF.

First off: 1 cm = .01 m

I can't see your picture. But you are right the basic idea for field values is vector addition.

The rest of it ... I can see no drawing. I'm not exactly sure what you are doing math wise.

Hootenanny
Staff Emeritus
Gold Member
Alright.
The Question states:

The drawing shows an equilateral triangle, each side of which has a length of 2.00 cm. Point charges are fixed at each corner, as shown. The 400 mC charge experiences a net force due to the charge q A and qB. This net force points vertically downward in the drawing and has a magnitude of 405 N. Determine the magnitude and algebraic sighs of the charges qA and qB.

This is what I did:

(1) I know that centimeters are supposed to be converted to meters
2.00*10-6
(2) Two vectors A & B are added together to produce a third vector C = A + B .
(3) When the triangle formed is not a right triangle:
the magnitude of the vector sum can be found by using the Law of Cosines or the Law of Sines

meaning:

4.00*10-6
-----------------------
1

so now I am lost
HELP!!
Welcome to PF,

Firstly, $2cm = 2\times10^{-2}m$ and not $2\times10^{-6}m$. Secondly, have you tried splitting the vectors into horizontal and vertical components? Since the net force acts downwards, you know that the horizontal components must sum to zero.

thank you
alright this is where my problem is
if: A B C
----= ----- = ---
sine(θA) Sine (θB) Sine(θC)

this is an equliateral triangle meaning:
2*10^-2 2*10^-2 2*10^-2
---------= ----------- -----------
1 1 1

and that is why I am stuck,.

Hootenanny
Staff Emeritus
Gold Member
thank you
alright this is where my problem is
if: A B C
----= ----- = ---
sine(θA) Sine (θB) Sine(θC)

this is an equliateral triangle meaning:
2*10^-2 2*10^-2 2*10^-2
---------= ----------- -----------
1 1 1