A physicist found that a force of 0.72N was measured between two charged sphere. The distance between the sphere was 0.90 m. Calculate the force between the two sphere when the distance was decreased to 0.50 m.

What you can use is coulomb's inverse square law that states that force varies inversly as the square of distance. It menas that the force is calculated by dividing by distance squared. what this does is essentially as the distance doubles, the forces is reduced by a factor of 4 or can be divided by 4. Force at 0.5m=2.3328N. And brings you ti the answer