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From: George A. Heintzelman (georgeh_at_[hidden])
Date: 2001-08-30 12:11:00

> Simple first reason, what is the conversion of kilograms to pounds? 2.2
> pounds to the kilogram right? WRONG! There is no conversion between the two,
> kilograms measure mass ( a constant ) and pounds measure weight ( a
> relative measure of mass * gravity ). If the quantity library allows both
> systems some compiler manufacturer will implement that wrong conversion and
> it will inevitably sneek into some calculation where it does matter.

A correct quantity library will of course supply conversions between
kilograms and slugs (the English unit of mass; a mass of 1 slug weighs
~32 pounds at nominal Earth-gravity) and Newtons and pounds
respectively. On the other hand, since pounds are frequently conflated
with mass in colloquial usage, it would probably not be inaccurate to
supply the constant pound_mass = 1/32 slug (= 1/2.2 kg).

Of course, a correct quantity library will quickly detect an error in
useage, as any attempt to use something measured in the force unit of
'pounds' where a mass was required will trigger a compile-time error.

Any quantity library desiring to be useful to US engineers, craftsmen,
and builders will *at least* have to supply instances of the constants
'pound' and 'inch' and so forth. Thus this error is potentially present
in either case. Most likely a library will also have to supply a value
for 'g', the earth's acceleration due to gravity, in order to be useful
for earth-bound systems anyway.

I don't think that this complaint is a reason to restrict users to SI


George Heintzelman

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