|Why is the outward force in a loop-the-loop a "fictitious" force? Why isn't it a "real" force?
A real force causes acceleration. If the outward "fictitious" force on a circling object were "real," that object wouldn't circle. It would accelerate outward. When you swing an object around on a string, you feel the object pulling outward on the string. But it isn't itself being pulled outward by anything. What you're feeling is the object's inertia trying to make it travel straight. The inward force you're exerting on it isn't opposing some real force, it's causing the object to accelerate inward.