# Homework Help: Direction and magnitude question again

1. Sep 9, 2007

### excelsion

1. The problem statement, all variables and given/known data
A charge of -6mC is placed at each corner of a square .1m on a side. Determine the magnitude and direction of the force on each charge.
upper left is 1, bottom left is 2 upper right is 4 bottom right is 3
http://img208.imageshack.us/img208/7318/payxhbx6.jpg [Broken]

2. Relevant equations
F=k (Q1Q2)/r^2

3. The attempt at a solution
sigh im back again..
9 X 10^9(6 X 10^-3 X 6 X 10^-3/.1^2) and i got
3.24 X 10^7

since all the charges are the same the answer is going to be the same for all the charges so F14 is also 3.24 X 10^7.

Now i break F12 into x and y components:
since 1 and 2 is repulsive there is no x component so the y component is is +3.24 X 10^7N

as for F14 there is no y component so x is -3.24 X 10^7.

now i do resultant formula:
total x is -3.24 X 10^7 and total y is +3.24 X 10^7
so the resultant should be... 4.58 X 10^7 right the book get the answer as 2.96 X 10^7

where did i go wrong?

Last edited by a moderator: May 3, 2017
2. Sep 9, 2007

### learningphysics

You didn't do F13.

3. Sep 9, 2007

### excelsion

F13 affects it too??!! wow how did i miss that ok so i do F13 is the same thing but the x and y comps are different:
3.24 X 10^7cos45=2.3 X 10^7
3.24 X 10^7sin45=2.3 X 10^7

the x component is negative and the y is postive since its moving north west..
now total x is -5.72 X 10^7 and total y is 5.53 X 10^7 then i do resultant formula and i get... 7.95 X 10^7 argghh what did i do wrong books answer is 2.96

4. Sep 9, 2007

### learningphysics

For F13 the total force is not 3.24x10^7... because the r isn't 0.1.

Still the force is going to be higher... does it say 2.96x10^7, or 2.96x10^8?

5. Sep 9, 2007

### excelsion

its 10^7
and i did the F13 over:
9 X 10^9(6 X 10^-3 X 6 X 10^-3/.14^2) and i get 1.63 X 10^7
find the component:
1.63 X 10^7cos45=1.15 X 10^7
1.63 X 10^7sin45=1.15 X 10^7

now the total X is
-1.15X10^7 + 0 + -3.24X10^7 = -4.39 X 10^7

total y is
1.15X10^7 + 0 + 3.24X10^7= 4.39 X 10^7
making the resultant:
6.2 X 10^7 ... why cant i get this thing!

6. Sep 9, 2007

### learningphysics

You sure the question is exactly as you've posted it? I'm getting the same answer as you.

Last edited: Sep 9, 2007
7. Sep 9, 2007

### excelsion

physics is making go mad im sorry... the NE and SW charges are positve...so let me try this again..
F12 is 3.24 X 10^7
x comp:0
y comp:-3.24 X 10^7

F14 is 3.24 X10^7
x comp:+3.24 X 10^7
y comp:0

F13 is 1.63 X 10^7
x comp:-1.15 X 10^7
y comp:1.15 X 10^7

Total X is 2.09 X 10^7
Total Y is -2.06 X 10^7

Resultant is 2.96 X 10^7(Finally got it)
and angle is found by doing tan inverse total y/total x and that is -45 deg
subtract that from 360 give me 315 degs pointing towards middle of square!

thanks for your patience lol see you tomorrow night most likely!