Once again in this chapter, we see the use of kepler’s third law to estimate the mass of supermassive black holes. in the case of ngc 4261, this chapter supplied the result of the calculation of the mass of the black hole in ngc 4261. in order to get this answer, astronomers had to measure the velocity of particles in the ring of dust and gas that surrounds the black hole. how high were these velocities? turn kepler’s third law around and use the information given in this chapter about the galaxy ngc 4261—the mass of the black hole at its center and the diameter of the surrounding ring of dust and gas—to calculate how long it would take a dust particle in the ring to complete a single orbit around the black hole. assume that the only force acting on the dust particle is the gravitational force exerted by the black hole. calculate the velocity of the dust particle in km/s.