Find q1 and q2?

Question

Two charges q1 and q2 have a total charge of 9uC. When they are separated by 0.5m, the force exerted by one charge on the other has a magnitude of 8mN. Find q1 and q2 for each of the following situations.
a) both are positive so that they repel each other
b)one is positive and the other is negative so that they attract each other

Answer 1

The equation you are looking for is the classic:
F=k(q1)(q2)/r^2

where:
F is the magnitude of force exerted,
q1 is the charge on one body (absolute value),
q2 is the charge on the other body (absolute value),
r is the distance between them,
k is approximately 8.988×10^9 N m^2/C^2 and is the electrostatic constant or Coulomb force constant

If you use the signs of the charges instead of absolute values, then the Force will be repulsive oriented, that is to say if the final value is positive then the force is repulsive and if the final value is negative then the force is attractive.