Fnet is not MG unless gravity is the lone force on the object.

Fnet is not mg if there are forces like ropes or normal forces acting on an object. I had no idea why, when you are told a rope is pulling a bucket up with a force of 105 N, you can say the net force on the bucket is Mg, but I think see the problem now. You think a and - g are interchangeable. a =-g is only true in free fall when gravity is the only force. Too many FORMULAS, too little understanding. Clearly you still do not know that Fnet is the vector sum of the forces on an object even when this fact is printed at the beginning of your test, or you do not understand that things that have forces on them other than gravity do not accelerate at -g. By now you should have noticed that not everything on earth is accelerating downward at -g. Since this course is about the real world and the real world is full of objects not in free fall you do, now , I hope, realize that a does not = -g in general and certainly not when a rope is pulling upward with a force greater than Mg.