# Homework Help: Charged particles - magnitude of force

1. Jan 30, 2006

### hoseA

http://img77.imageshack.us/img77/5310/chargedparticles7of.png [Broken]

What is the magnitude of the force on a
-1.2 muC charge placed at a point 1.6 cm to the left of the
middle charge?? Answer in units of N.

This is what I did:

I used Fx = k*|Q|*(qx/r^2)

Q= 1.2*E-6

I plugged in 8.6*E-6 into the above equation (keep in mind i used 3.7cm-1.6cm =2.1 cm --> .021m) and got F1=210.320898

I plugged in 4*E-6 into the above equation (i used .016m) and got F2=168.5165625

I plugged in -1.2*E-6 into the above equation (i used .016+.032=.048m) and got F3 = -5.61721875

Then for F final = -F1 + F2 - F3 = -36.187

Since it asks for magnitude I plugged in 36.187. It came up as wrong... where did i go wrong??

Last edited by a moderator: May 2, 2017
2. Jan 31, 2006

### hoseA

It would be great if someone can offer any help within the next 3 hours.

3. Jan 31, 2006

### jdstokes

According to the way you've defined F1, F2 and F3, your net force should be

Fnet = -F1 + F2 + F3

which comes out as 47 something newtons.

4. Jan 31, 2006

### hoseA

Wow, that was it. Thanks A LOT!!!! People like you make this site awesome. Thanks again.