My book says r^3/t^2 was found constant by Kepler. What this constant is called? How is acceleration equal to v^2/r in case of circular motion? Why is rate of change of momentum or say product of mass and acceleration equal to force? Why not was it ma^2? How was it proved that it is equal to ma? How newton showed that a spherical body of uniform density behaves as if whole of its mass is concentrated at its center? Also, why is it that a symmetrical body of perfect density balances about the point of center of gravity? What are the latest and advanced means to calculate the value of acceleration due to gravity? How do we estimate mass of a double star? These questions remain unanswered in my book. Maybe because they are little tough to understand. Please keep the replies as simple as you can so that I have no problem in understanding them. Thanks for reply!