Here are two things I was taught in college that were absolutely wrong.
- The Coriolis effect explains why bathtubs drain clockwise in the northern hemisphere.
- The Bernoulli effect explains how planes fly.
These are not things that scientists believed at the time but were later disproved. They’re simply myths. (No doubt I’ve passed on a few myths to students over the years. If I can think of any, I’ll retract them here.)
The Coriolis effect does explain why cyclones rotate one way in the northern hemisphere and the opposite way in the southern hemisphere. The rotation of the earth influences the rotation of large bodies of fluid, like weather systems. However, a bathtub would need to be maybe a thousand miles in diameter before the Coriolis effect would determine how it drains. Bathtubs drain clockwise and counterclockwise in both hemispheres. Random forces such as sound in the air have more influence than the Coriolis effect on such small bodies of water.
The explanation I learned in college for how airplanes fly involves the Bernoulli principle. The shape of a wing is such that air has to travel further over the top of the wing than under the bottom of the wing.
Since air particles going across the top and bottom of the wing arrive at the trailing edge at the same time, the particles going over the top travel further and so are spread further apart. This results in lower pressure over the top of the wing and lifts the airplane.
There are a couple problems with this explanation. When the air particles split at the leading edge, why should the ones that go over the top and the ones that go under the bottom arrive at the trailing edge at the same time? In fact, they don’t. Also, while the Bernoulli effect does explain part of the lift on an airplane wing, the effect is an order of magnitude too small to account for why airplanes fly.
If the Bernoulli principle explains lift, how can an airplane fly upside-down? Wouldn’t the Bernoulli effect suck the plane down rather than lifting it up when you turn the wing over?
Here’s a set of recordings from a lecture that debunks the Bernoulli effect myth and explains the real reason airplanes fly. Why airplanes fly: a modern myth. Part I, Part II. There will be a Part III that hasn’t been released yet.
Update: Apparently these links have gone away.