# Homework Help: Trouble with electric force question

1. Dec 20, 2013

### con31773

1. The problem statement, all variables and given/known data
I'm having a little trouble with an electric force question I have been working on. It seems simple, but my answer keeps coming out incorrect. Using the graph, deduce the electric force vector on charge q3 by q1/2. In case it is not clear, q3 lies on point (2,2)

2. Relevant equations

F=(kq1q2)/r^2*$\hat{r}$

3. The attempt at a solution
Okay, well since electric force vectors simply add to give resultant force I can deduced the individual vectors and sum, as such.

F(1/3)=(kq1q3)/r^2*$\hat{r}$(1/3)

and the same applies to the second charge

F(2/3)=(kq3q2)/r^2*$\hat{r}$(2/3)

then I deduced r for both. Simply being 2 for q(2/3), and 5 for q(1/3) (from pythagoras)
Then, the unit vectors, as (2$\hat{i}$+$\hat{j}$)/5 for q(1/3)
and $\hat{j}$ for q(2/3)

Put it together I got
F(1/3)=((kq1q3)/25)(2$\hat{i}$+$\hat{j}$)

F(2/3)=((kq2q3)/4)$\hat{j}$

Evaluate and sum, I got, 0.367$\hat{i}$-0.412$\hat{j}$

which of course is not correct. I'm sure there is a mistake in there but can not find it. Would be greatful for some fresh eyes to tell me where.

2. Dec 20, 2013

### TSny

Hello con31772 and welcome to PF!

Oops. Check what you got for r for q(1/3).

3. Dec 20, 2013

### con31773

Oops. Check what you got for r for q(1/3)

Haha, thanks very much. Could not find my mistake at all.