In order to compare the eletrostatic forces, let's calculate each one using the formula:
[tex]F=k\frac{q_1q_2}{r^2}[/tex]
Where k = 9 * 10^9, q1 and q2 are the charges and r is the distance between them.
So we have:
[tex]\begin{gathered} q_1=3\cdot10^{-6},q_2=5\cdot10^{-6},r=2\colon \\ F=9\cdot10^9\cdot\frac{3\cdot10^{-6}\cdot5\cdot10^{-6_{}}}{2^2} \\ F=0.03375 \\ \\ q_1=3\cdot10^{-6},q_2=3\cdot10^{-6},r=2\colon \\ F=9\cdot10^9\cdot\frac{3\cdot10^{-6}\cdot3\cdot10^{-6_{}}}{2^2} \\ F=0.02025 \\ \\ q_1=3\cdot10^{-6},q_2=3\cdot10^{-6},r=3\colon \\ F=9\cdot10^9\cdot\frac{3\cdot10^{-6}\cdot3\cdot10^{-6_{}}}{3^2^{}} \\ F=0.009 \\ \\ q_1=5\cdot10^{-6},q_2=5\cdot10^{-6},r=3\colon \\ F=9\cdot10^9\cdot\frac{5\cdot10^{-6}\cdot5\cdot10^{-6_{}}}{3^2} \\ F=0.025 \end{gathered}[/tex]
So the weakest force is the one in the third case (bottom left option): charges of -3 uC and +3 uC, and distance of 3 meters.
