Example: Three stars, each of mass m = 50 x 10^{20} kg, are situated on the vertices of an equilateral triangle of side a = 8 x 10^{3} km. The only force acting on them is the gravitational force. Due to some cosmic event, all the three stars are simultaneously set in motion. The three stars move in a circle, maintaining their original separation and configuration. Find:
 the velocity of each star; and
 the time period of the circular motion.
Solution:
The situation is shown in the schematic diagram above with the three stars labeled A, B and C. Let each star attract the other by a force F'. Then The resultant force on any star, say A, is For the circle, the radius r is given by Assume star A is moving with velocity v. The centrifugal force is balanced by the gravitational force of attraction. A force balance on star A gives Thus, the velocity of each star is Substituting G = 6.67 x 10^{11} N m^{2}/kg^{2}, m = 50 x 10^{20} kg and a = 8 x 10^{6} m, the velocity v = 204.18 m/s.
b. The time period T of the circular motion is readily calculated from Substituting v = 204.18 m/s and r = 4.62 x 10^{6} m, the time period T = 142.13 x 10^{3} s.
