What is the radius of a black hole with a mass of 10 M⊙? Suppose you place a bowling ball in orbit around this black hole at a distance of 90 km from the center of the black hole. Use Kepler’s third law as modified by Newton to calculate the bowling ball’s orbital period (remember to convert kilometers to astronomical units). How long is this in seconds?
1) Schwarzschild's radius =2GM / c^2 =267 Km for 90 solar masses.
2) Sorry, I have forgotten the formula for "orbital velocity" !! . Too lazy to look it up!. It must be in your Physics, or Astronomy textbook!.