PFE017: Centrifugal Motion

The notion of a centrifugal force is often rather poorly understood. In high school, I was told, explicitly, that there is no such thing as a centrifugal force. Unfortunately, I passed this information on to others before I was corrected.

A centrifugal force is only felt from the point of view of someone moving in a circle. A car going around a turn. One of those carny rides.

You feel... pushed outwards. It feels like, if there were no wall, or side to your car, you might just go flying straight out. So there must be some force going out.

At this point, some people might tell you that there is no such force. None of the four fundamental forces (gravity, electromagnetism, strong and weak nuclear forces) can be tied to the present phenomena.

In general, physics is usually conducted in what is known as an "inertial reference frame" or a non-accelerating reference frame. An accelerating reference frame would be from when you step on the gas until your car maxes out its speed. Or, for example, on a spiny carny ride.

The reason why these situations tend to be avoided is because they add unnecessary complications. The study of forces is the study of accelerations, and adding additional accelerations adds a sort of "fictitious force", although I find that term is rather misleading simply because we are on the earth. And the earth rotates on its axis. The the earth orbits the sun. And the sun orbits the galaxy. So clearly we are in a non-inertial reference frame.

Effects from the earth spinning are measurable, in theory, but small. The main practical difference is that if you hang a plumb bob (weight on a string) it will deflect from the center of the earth. That is, the direction that we think of as "down" is not exactly toward the center of the earth as we would predict from gravity. This can give rise to deflections of nearly a tenth of a degree depending on latitude (or about 2 inches in 100 feet) (more extra credit! (pdf)). So then why don't buildings fall over all the time? Simply put, the forces felt on the plumb bob, "fictitious" or not, are the same felt throughout the whole building.