1. Jan 12, 2009

### ScottL

1. The problem statement, all variables and given/known data

In the figure 21-29, particles 2 and 4, of charge -e, are fixed in place on a y axis, at y2 = -7.20 cm and y4 = 16.6 cm. Particles 1 and 3, of charge -e, can be moved along the x axis. Particle 5, of charge +e, is fixed at the origin. Initially particle 1 is at x1 = -9.84 cm and particle 3 is at x3 = 9.84 cm. (a) To what x value must particle 1 be moved to rotate the direction of the net electric force net on particle 5 by 30.1o counterclockwise? (b) With particle 1 fixed at its new position, to what x value must you move particle 3 to rotate net back to its original position?

....................|....................
....................|....................
....................4....................
....................|....................
....................|....................
-------1------5-------3------
....................|....................
....................|....................
....................2....................
....................|....................

I tried to represent the picture that was provided as accurate as possible.
here is the actual picture that I was provided with.
http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c21/fig21_38.gif

2. Relevant equations
Ok I know that I am supposed to use
FE = k(Qe*qp)/r2)
I can get the force that each one has on point 5 just by using the equation above but then I am stuck.

3. The attempt at a solution
I have not attempted the solution yet because I don't know where to start.

2. Jan 12, 2009

### IPart

Okay. I assume you have heard about the principle of superposition, i.e., that you can add up (vectorially) the force due to each of the charges on any one given charge. Using this principle (and Coulomb's Law), first compute the direction of the net electric force on particle 5 when particle 1 is at its initial position. Note that force is a vector: so you will have to specify the direction of the net force (as well as its magnitude) when you state the answer. Once you have done this, more help will follow!

3. Jan 12, 2009

### ScottL

Ok Since Fx net = 0 the only ones to take into account are the y directions.
I have come up with Fnet = -F52 + F54 = |3.622*10-26|

And the direction is towards the -y correct?
Or am I completly off base?

Last edited: Jan 12, 2009