Mass and Weight
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less."--Lewis Carroll
We'd be in a bad way without words, but sometimes they can be as much of a barrier as a help to understanding. A lot of the problems people have with science is due to jargon -- words defined to mean something very specific in a particular field, but often not comprehensible to anyone outside that field. The worst kind of jargon is a word used to mean one thing in ordinary conversation and defined to mean something slightly -- but critically -- different in a particular field. Take "weight", for example.
Everybody knows what weight means. "Oh, that this too, too solid flesh would melt" weight. Get the weight off your feet. A ten-pound sack of flour. Two point two pounds equals a kilogram. Then along comes a physics teacher who says "No, weight is a force, and pounds measure force, but kilograms measure mass, so you cannot express pounds as kilograms or vice versa." Is it any wonder beginning physics students get confused? In physics jargon, weight is the force of gravity acting on mass.
The fact of the matter is, we use "weight" in ordinary English to include both mass (the too, too solid flesh) and the force the earth's gravity exerts on that mass (the weight on your feet). In order to do this, we ignore the fact that the earth's gravity actually pulls slightly harder at the poles than at the equator. But that difference is very small, and by pretending that it does not exist we have the convenience of using the force the earth exerts on a mass to measure the amount of the mass. A kilogram may not be 2.2 pounds, but the earth's gravity does indeed pull on a kilogram of mass with a force of 2.2 pounds. As a result, people in the United States hardly ever refer to the English unit of mass, the slug, while people in most of the rest of the world use kilograms in a casual way to refer to gravitational force. (The correct unit for this would be the newton.)
When I buy a forty pound sack of dog food, I expect it to feed my dogs for a certain length of time. In other words, it is the mass, not the weight, that I am purchasing. But if I leave the sacks in the back of my car to improve traction in winter, I am making use of the weight that gravity, acting on the mass in the sacks, puts over the rear wheels of the vehicle. It's also the weight that concerns me as I stagger into the house with the sacks.
If we ever have a lunar colony, with people moving back and forth between our earthly gravity and that of the moon (which is only one-sixth as strong as Earth's), we may start to develop an intuitive distinction between mass (which remains the same between here and the moon) and weight (which is only a sixth as much on the moon). Because it would have six times the mass, a 10 pound sack of flour on the moon would bake six times as much bread as a sack weighing 10 pounds on earth. (Goods labeled in pounds on earth would have to be relabeled on the moon.) A five-kilogram sack, on the other hand, would be exactly the same sack as on earth. (On the moon, however, it would be a mere featherweight to carry)
Perhaps it would be easier if the physicists had chosen a completely new term for gravitational force, instead of using weight. The fundamental confusion between two things which in our everyday experience are always linked together, however, is likely to persist as long as we all live on a single planet.
