I am asked to determine how certain modifications to the variables in Coulomb's equation will effect the resultant force:
$F=k\frac{Q_1Q_2}{r^2}$ The question asks me what will happen with $Q_1$ doubles, and I determine $F$ is doubled. Then I am asked what happens when $Q_1$ and $Q_2$ are doubled, and I get $F$ is quadrupled. But then I am asked what happens when $r^2$ is tripled and I come across some confusion. I calculate $F$ is one third, using $1m$ for $r$ and then solving the equation once for $r=1^2$ and then $r=(1^2)3$, and comparing the resulting values in magnitude. The answer states that $F$ is a ninth.
Additionally, I am asked what will happen to $F$ when $Q_1$ and $Q_2$ are doubled and $r^2$ is halved. I am hoping that someone cant hint me in the right direction.