How do I calculate the gravitational potential energy between two objects that are separated by near to infinite distance? I would like to know if the two objects are kept at near infinite distance, then at what kinetic energies would they come and collide into each other.
The way to calculate gravitational potential difference is $$U = -G m_1 m_2 (1/r - 1/R).$$ Where in I can enter R = infinity. However, when I put r = 0 it gives me infinite energy, which doesn't seem possible. That would mean r will have a positive value. What is the value?
(This question is out of curiosity and not for any assignment.)