According to the Bible (in the book of Genesis, chapters 9-11), the world was once covered by water, in a great flood. A man called Noah built a big ship (called the “ark”) that he used to rescue his family and all the species of animals. After the flood, his descendants lived in a place called Shinar. They all spoke the same language, since they were members of the same family, and the community flourished. After a while, they decided to build a tower (called the “Tower of Babel”) that was so tall that it would reach up to heaven. They thought that, when they got to heaven, God would become redundant. Not surprisingly, God didn’t like the idea of being made redundant. So, he made everybody speak a different language, so they couldn’t communicate on the project. Just to make sure, he scattered them all over the world. The result of this was that Noah’s descendants were no longer able to collaborate and so stopped building the tower.
If you’ve been following this blog, you will probably have realised that science attempts to define words and the units that we use to measure things very precisely. So, words like force (post 16.13), energy (post 16.21), power (post 16.23) and frequency (post 16.14) have meanings that are very carefully defined, to ensure that we all use them in the same way. Similarly, the words “speed” and “velocity” don’t mean exactly the same thing (posts 16.2 and 17.4). And we measure force in newtons (post 16.13), energy in joules (post 16.21), power in watts (post 16.23) and frequency in hertz (post 16.14), to make sure that we can share our ideas. The reason is to ensure that we all understand each other and can share ideas in a way that became impossible for the descendants of Noah.
Unfortunately, it doesn’t always work!
One reason is that, although there are no clear boundaries between physics, chemistry and biology (post 16.41), the three subjects developed independently. As a result, different areas of science sometimes use the same word to mean different things. For example, in physics the word “vector” has its mathematical meaning – something that has both quantity and direction (post 17.2). In biology, a vector is a living thing that spreads a disease or parasite – so a mosquito is a vector for malaria. Fortunately, these two uses are so different that they are unlikely to lead to confusion. However, physiologists sometimes refer to “oxygen tension”; here the word “tension” has nothing to do with stretching (the use described in post 16.49) but is used to mean “pressure” (see post 17.5) – even though the two words have completely different meanings in physics. So, when a doctor talks about hypertension he means high blood pressure. Surgeons use the word “fracture” when a bone breaks and “rupture” when a ligament breaks. Materials scientists use the word “fracture” when anything breaks. This wouldn’t normally matter but when the two collaborate, which is increasingly common, this difference in usage can cause confusion.
Another problem is that words are sometimes used incorrectly in the scientific literature. It is not uncommon to find the phrase “body weight” (measured in kg) in the medical literature when the authors really mean body mass. I have often found the word “elasticity” used to mean stretchiness. However, elasticity means the ability to store energy for use in recoil (see post 16.49). The opposite of stiffness is compliance.
Sometimes confusion is caused by carelessness. For example, nutritionists frequently use the word “calorie” when referring to a kilocalorie (a thousand calories); a calorie is an old-fashioned unit used to measure heat energy and is equal to 4.2 J. Sometimes physicists use the word “frequency” to mean both “frequency” (post 16.14) and “angular frequency” (post 17.12). I used to find this very confusing, until I developed the habit of looking at their equations to discover exactly the intended meaning. If people chose their words more carefully, the meaning would be clear.
Finally, two different systems of units, for measuring mass, length and time (post 16.12) have developed. In this blog, I use SI units where mass in measured in kg, length in m and time in s (post 16.12). From these basic units we define the newton (to measure force, post 16.13), the joule (to measure energy, post 16.21) and the watt (to measure power, post 16.23). An earlier system, that is still sometimes used in the USA, uses the gramme (g) as the unit of mass, the centimetre (cm) as the unit of length and the second (s) as the unit of time. In this system, the unit of force is the dyne (abbreviated to dyn) which is 1 g.cm.s-2; the unit of energy is the erg which is 1 dyn.cm. This is called the cgs system.
A related problem is that forces are sometimes described by the force that a mass would experience in the earth’s gravitational field (posts 16.16 and 16.17). So 1 kg-force is the force experienced by a mass of 1 kg on the surface of the earth. Even worse, you can sometimes see forces measured in the units of lb-force where lb is the abbreviation for an antiquated British (and American) unit of mass – the pound. And worse still, the magnitude of the gravitational field varies over the surface of the earth (post 17.9).
It would be much better if everyone used SI units and never used units like kg-force!
Why do these problems exist? I think it’s a combination of different subjects developing independently, ignorance and carelessness. But, if you believe the Bible, perhaps it’s inevitable after Noah’s descendants, who would be our ancestors, tried to build the Tower of Babel!