Why is centrifugal force labeled a fictitious force?”
Start with Newton’s definition of a “force”, F=ma. If something isn’t moving, it’s not experiencing a force. And remember that a is a vector. That is, it points in a particular direction. Consider what’s happening if you’re standing on a spinning space station, the kind where they’re using “centrifugal force” to give the thing a floor. Which way are you moving at any given instant? You’re moving sideways, parallel to the floor. That’s Newton’s first law: an object in motion tends to stay in motion. If the spaceship is spinning to the left, then left is the direction that you’re moving at each instant. But, you are changing direction, at any instant, because you’re not actually flying out to the left. The floor of the space ship catches you before you fly off to the left, changing your direction. That change of direction is acceleration, a. And the direction of that acceleration is “in”, towards the center of the spacecraft. So… we’ve got acceleration in, and therefore a force, but it
