Electric forces questions

[tonithetiger]

i was absent for a couple days and i have absolutely no idea whats going on. Help with any of the questions would be amazing.


http://i132.photobucket.com/albums/q38/margaritamarie/physics.jpg


for #1 first i found the diagonal distance equals \sqrt{98}
then i used the formula F=KC x q1 x q2/ r^{2} to find the electric force between the 8nC charge and the two others:
8.99E9(8E-6 x 7E-6)/ 98 = 5.14E-3
and 8.99E9(8E-6 x 2E-6)/ 98 = 1.47E-3

after that i had no clue, except i know #4 is 0

[CompuChip]

Welcome to PF.

Don't forget to include the direction when calculating the force. The formula you gave is actually a vector equation, and to get the magnitude and direction right you should add the two forces as vectors.
Also, when writing down a final answer, include units (if they ask you to calculate force, your answer will be something like: F = ... N in the x-direction, or \vec F[N] = \cdots \hat{x}).

You have learned to work with vectors already, is that correct?