Treacherous Words
Today, treacherous words. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.
______________________
We all talk in two worlds at the same time. A technical world where words have one set of meanings. Or everyday speech where those same words have different meanings. Suppose I tell you that the average speed of the air molecules whizzing around us in our room is just over a thousand miles per hour. But that their average velocity is zero.
When you and I converse, speed and velocity are synonyms. But when we deal with those molecules, we need one word for how fast they move. And a different word that combines their speed with the direction they move. Velocity is a vector quantity – a speed in some particular direction. And all those competing directions of speed average out to Zero.
Velocity and Speed distributions in, say, the air around us. Note that the velocity maximizes at zero while the speed (which can never be negative) vanishes at zero. (Image from the Tien Lienhard source)
We thermodynamicists are a most bothersome breed. Always bending the common tongue to our needs! I won’t labor you with all the problems surrounding the word entropy. You’ve likely been beaten about the ears with all its complications. Instead, let’s look at a simpler word: “heat”.
Heat is both a verb and a noun in everyday speech. But we use it only as a verb. Let’s see how that works: We heat a pot of water. Then the water contains new thermal energy. Our physics would get terribly messy if we didn’t clearly distinguish between energy being transferred and energy contained. For us, energy is heat, only when it’s in motion.
Technical language has to go where our common words cannot take us. Suppose we want to explain the strength of materials. Words like stress, strain, yield, failure, shear – each already has a meaning in common speech that could never explain what goes on in various materials. Did you know that we can identify nine different stresses at any one point in a material? We co-opt all those common words to describe how, say, a certain steel reacts, as we load it in different ways.
The so-named stress tensor showing the nine stresses that can act upon any point in a solid or fluid. (Image courtesy of Wikimedia Commons)
Of course the language of science can run amok if we aren’t careful. Suppose I stood, whirling a rock on a string. We were once taught to distinguish centrifugal and centripetal forces – one on my hand and one on the rock. Groan! If that made sense, we’d also have to invent two forces whenever I sit in a chair – one force on my bottom and a different one on the seat. No need to make our technical language more complex than it has to be.
You listeners work in all kinds of arenas, construction, medicine, science, law, ... I’m sure you too use all sorts of language that might sound like plain English at first blush. I spoke at the outset of treacherous words. I hope we can keep our technical language as close to being understandable as possible – in a world that gets more complex every day.
I’m John Lienhard, at the University of Houston, where we’re interested in the way inventive minds work.
(Theme music)
C. L. Tien and J. H. Lienhard, Statistical Thermodynamics, Hemisphere Pub. Co. 1971. Here we talk about velocity and speed of molecules and present the image used in this episode. You may read this book online here. The image in this episode is taken from this source.
See Wikipedia articles for both Centrifugal and Centripetal forces. “Centripetal” retains some currency despite its vanishing use.
Here is a kindred episode that I did many years ago. Specialized Language | The Engines of Our Ingenuity
This article treats the “stress tensor” (the nine stresses) that I mention in this text.
For more on heat transfer and energy, see Chapter 1 of J. H. Lienhard V and J. H. Lienhard IV, A Heat Transfer Textbook, Dover. Available free of charge online here.
This Episode first aired on July 29, 2026.