Dave Van Domelen
Kansas State University
Physics Education Research Group
Last revised 1/13/08
Introduction and Motivation
At some point in their lives, most people hear about the Coriolis force, whether in reference to weather patterns, sea currents or, most prosaically, which way water flows down the sink. Unfortunately, while many have heard of it, few understand it well enough to explain it without resorting to vector equations. And textbooks will generally either go straight to the equations, or not really explain it at all.
First, a bit of terminology. The Coriolis force is actually only part of the overall effects of being in a rotating system, a result of how the math was worked out. So I'm generally going to talk about frame effects rather than Coriolis in specific, because the distinction between what parts are or aren't Coriolis is sometimes confusing and not really important for this explanation.
So, what to do? This article intends to develop a means of explaining the Coriolis effect (and the overall frame effects) to people who haven't yet grasped angular mechanics. This explanation relies on linear quantities and uses rotational concepts infrequently.
The Basic Premises
There's really only three things that really need to be established right now, although there will be a few other concepts that will creep in later when we cover rotating spheres.
Two-Dimensional Frame Effects
Okay, let's start out simple. Why does standing on a rotating object cause mysterious forces to appear? The short answer is that our assumption of firm ground is incorrect, and what we consider to be "forward" or "to the left" changes as we spin around. For the long answer, click here.
Frame Effects on a Sphere
Now, without worrying about exactly why things stay on a sphere (a mix of gravity and the ground stopping you from falling too far), we tackle the more complicated task of looking at the Coriolis and centrifugal forces on the surface of a rotating globe like the Earth.
Putting It All Together
Now that the basics have been covered, what is it good for? Keep in mind, though, that on the Earth, the maximum acceleration due to Coriolis and centrifugal forces will each be about 1/300th the strength of gravity. So if there's any other forces involved that are comparable to gravity (like friction between your feet and the ground), there's a good chance frame effects will wash out.
Thanks to the readers of the Usenet newsgroups alt.fan.cecil-adams and misc.education.science for asking the questions which inspired the author to devise an explanation for the Coriolis effect. Thanks also to Donald Shabkie, who pointed out the importance of the Coriolis effect to aviators after seeing the original explanation online, to Steven Carson, who pointed out the references in Nature, and to Cleon Teunissen who pointed out some problems with the second version and inspired me to take another whack at the problem. Finally, work on the earlier versions of this explanation was supported in part by NSF grants NSF GER-9553460 and NSF DUE-9396205.
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