How Do Newton's Laws Explain Changes in Gravitational Forces and Orbital Ratios?

[sarahhhh]

Homework Statement

1. What happens to the gravitational force between two masses when the distance between the masses is doubled?

2. According to Newton’s version of Kepler’s third law, how does the ratio (T^2/r^3) change if the mass of the sun is doubled?

 

[hage567]

For the first one, what's the equation of gravitational force? What happens if you double the distance and keep everything else the same? Tell us what you think.