The electrostatic force between two charges Q1 and q is given by
[tex]F=k_e \frac{Q_1 q}{r^2} [/tex]
where
ke is the Coulomb's constant
Q1 is the first charge
q is the second charge
r is the distance between the two charges
Re-arranging the formula, we have
[tex]q= \frac{F r^2}{k_e Q_1} [/tex]
and since we know the value of the force F, of the charge Q1 and the distance r between the two charges, we can calculate the value of q:
[tex]q= \frac{(0.961 N)(0.67 m)^2}{(8.99 \cdot 10^9 N m^2 C^{-2})(+8.44\cdot 10^{-6}C)}=5.69 \cdot 10^{-6} C [/tex]
And since the force is attractive, the two charges must have opposite sign, so the charge q must have negative sign.
