Three spheres, with masses indicate, are initially far away from each other, and the gravitational system of the system is zero. The spheres are then brought together until each sphere is a distance r from the other two. What is the new gravitational potential energy of the three-sphere system

Why does the equilibrium position of the spring change when a mass is added to the spring? will the mass oscillate around the new equilibrium position of the spring or the previous position without a mass attached to the spring? if the equilibrium position of the spring changes by 20 cm (assuming no initial mass) when a mass is added to the spring with constant 4.9 kg/s^2, what is the mass of the object attached to the spring?