Question

The force between two stationary charged objects is measure to be 15.0 N. If distance between the objects is doubled, what is the new force between the two objects?

Answer 1

The force between two stationary charged objects is 

$$F=\frac{kq_1q_2}{r^2}$$

The new force:

$$F=\frac{kq_1q_2}{(2r)^2}=\frac{kq_1q_2}{4r^2}=\frac{F}{4}=\frac{15}{4}=3.75 N$$