Foolish at Physics

A science textbook tried to explain the fact that a shower curtain (unweighted and without magnets) will push toward you by applealing tp the Bernoulli principle. This claim is shown to be ignorant in “Fun in the Tub” by Howard P. Lyon:

Over the past few years, a half-page article called “Attack of the Shower Curtain” has been printed again and again in middle-school books published by Prentice Hall. It first appeared in the 1993 version of Motion, Forces, and Energy, one of the books in the Prentice Hall Science series. Since then it has been used in later versions of Motion, Forces, and Energy and in several successive versions of Prentice Hall Exploring Physical Science. … In their pedagogic note, Prentice Hall’s writers tell the teacher that this concept explains a shower curtain’s aggressiveness:

[The “Attack” article] helps students relate Bernoulli’s principle about pressure in fluids to a real-life situation. . . . The pressure exerted by the fast-moving fluid (water inside the shower) is less than the pressure in the surrounding fluid (air outside the shower). The greater outside pressure pushes the shower curtain into the shower . . . .

… In an effort to find out why shower curtains sometimes attack showerees, I observed the action of the curtain that adorns my own bathtub. … Clearly, the “Attack of the Shower Curtain” is a response to temperature. When the water falling from the showerhead is hot, it warms the air inside the shower enclosure and induces convection. The warmed air rises, escapes from the enclosure by flowing over the top of the curtain, and is replaced by cooler air that flows into the bottom of the enclosure. As this cooler air enters the enclosure, it pushes the lower margin of the curtain ahead of it, causing the lower part of the curtain to swing inward.

As Galileo would say, one needs to ‘make the experiment.’ And as Galileo said of Aristotle: “[F]or I am sure that he never took [the belief that there is no change in the heavens and celestial sphere, that things are perfect, timeless, and indestructible in the heavens and the celestial sphere] to be as certain as the fact that all human reasoning must be placed second to direct observation.” (p. 118, Discoveries and Opinions of Galileo, trans. Stillman Drake, Doubleday Anchor Press, Garden City, NY, (c) 1957 Stillman Drake.)