Question

calculate the netforrce on q2?
calculate the net force on q3?
q1=+3.0 MC
q2=+3.0MC
q3=-6.0MC


q1-------20cm-------------q2
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50 cm
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q3

Answer 1

Hey, just use Coulomb's Law

F = k Q1 Q2 / d^2

if like charges, repel
if opposite signs, attract

see:
http://en.wikipedia.org/wiki/Coulomb's_law

Answer 2

And remember, you're adding VECTORS. You can't just add the two numbers. They have directions as well.