2D physics forces for circular objects

I'm trying to work out the resultant forces when two circular objects interact.

Each object has known attributes: position; velocity; mass

The underlying formula needs to be able to calculate the force whatever the orientation of the positions are.

So, with two objects Obj1 and Obj2

Point1 = Obj2 - Obj1;

theta = arctan(Point1.x,Point1.y) - angle of the collision

sine = sin(theta)

cosine = cos(theta)

So now I need to change the coordinate system and work out the force then change back but i'm so lost =(((

Please help