Dutch Roll

Gene Nygaard wrote:

"Julian Scarfe" wrote in message ...
I spent a while writing the physics bit of the New Penguin Dictionary of
Science. The hardest part was knowing whether to be prescriptive (tell them
what the usage *should* be) or descriptive (describe what the common usage
*is*). It's a judgement call in almost every case -- for example, I had no
qualms about defining "weight" quite carefully to distinguish it from
"mass", even though many people say "weight" when they mean "mass".

You are confused if you think you made a correct "should be" call on
this.

Well, perhaps you missed the fact that he was writing definitions for
a dictionary of _science_ ?

There is no conflict, really, between the call he made, in that context,
and the everyday meaning you go on to defend, and quite convincingly so.

When we say our bag of sugar has, as it might be labeled in the U.S.,
a "net weight" of 10 lb (4.54 kg), where the pound is of course a unit
of mass officially defined as 4.5359237 kg, that is absoloutely
correct and proper, well justified in linguistics, in history, and in
the law.

Absolutely.

That's the original meaning of the word "weight," which entered the
English language meaning the quantity measured with a balance, used to
measure goods sold by weight in commerce. We measures mass, as that
term is used in physics jargon today, with a balance--not the force
due to gravity.

Not quite, though you have a point. What we measure with a balance
is the relationship of the force due to gravity of the object we
want to weigh, to that of a reference object of a known weight.
We are measuring relations between "weights", as the term is
understood in physics. To illustrate this, in the absence of gravity
we could not measure mass in this way (well, we might contrive a way
to use inertial forces, but we'd still be measuring forces).

Where you do have a point is in the sense that this method will give
consistent results whether performed on earth, on the moon, or in any
other gravitational field. Spring-based scales of course measure
absolute weight and will only give correct (mass) results in a
standard gravitational field eg. on the earth's surface.
In both cases we express the result in units of mass.

In other words, it isn't a case of us saying the wrong thing. We mean
to say "weight"; we mean "weight" in a quite legitimate and proper
meaning of the word; it just happens to be the same quantity that
physicists happen to call "mass" in their jargon--but we don't
normally "mean" something different from what we "say."

Agreed. It's not really a case of either being wrong. It's just that
in physics there is a need to differentiate and keep the two concepts
apart, while in everyday life there is normally no such need.

Most people (even physicists, I imagine), quite correctly and
appropriately, say "weight" in everyday contexts, when referring
to something that is really "mass", as the concept is understood
in physics. They are not saying anything different from what
they mean. They are merely applying a level of differentiation
of concepts, appropriate to the situation at hand.

5.7.4 The use of the verb "to weigh" meaning "to determine the mass
of," e.g., "I weighed this object and determined its mass to be 5 kg,"
is correct.

If the weight and the gravitation are known, the mass can be determined,
so the above sentence is quite correct in any context, even as strictly
understood in physics.

CV