Q&A: Black Holes

Q:
"The gravity of a black holes falls off inversely as the square of the distance?" Sorry what does that mean exactly?

A:
It means that as you get three times as far away, the gravity is nine times weaker, etc. This is a general rule discovered by Newton that applies to all objects that have mass. By balancing gravity with the acceleration due to orbital speed, it is possible to estimate the mass of the object producing the gravity. This is the way that astronomers weigh the Earth, the moon, stars, black holes, galaxies, etc.