In our software we use the term English
units to refer to the system of measurements used in the
United States. Some units, such as the gallon, are a bit different
in the English system than in the Imperial system, and that's
why we use the English system.
The Pound Is The Unit Of Mass In The Absolute
System Of Imperial Units.
If you're not all that interested in why we use pound as
the unit of mass you don't need to read any further. It's
an academic point, but one that gets brought up by users from
time to time. We wanted to answer that question and link to
some web pages that go into more detail.
Many of us learned that the only unit of mass in the Imperial
system is the Slug,
but that was never true. Long before the Slug was invented,
was used as the unit of mass and the Poundal
as the unit of force. In the Imperial Gravitational system
the slug is the unit of mass and the pound is the unit of
force. However, in the Imperial Absolute
system the pound is the unit of mass and the Poundal
is the unit of force. In both these systems Newton's second
law can be written as...
F = ma
Why not use slugs for mass? Because in most engineering
the Imperial unit of momentum is the Pound Foot Per Second
rather than the Slug Foot Per Second. To get Pound Foot Per
Second from the momentum equation requires using the Pound
as the unit of mass. Furthermore, the unit of bullet weight
is the Grain,
which is derived
from the Pound, not the Slug. If you weigh bullets in
grains then momentum should be measured in Pound Foot (Feet)
Per Second, which it commonly is.
The creator's of the Metric system wisely chose the Kilogram as both
the unit of weight and the unit of mass. To do this and still have Newton's
second equation work out without applying the acceleration due to gravity
they invented the Newton
as the unit of force. It turns out the Imperial system did the same thing
long ago by inventing the Poundal for use as a unit of force.
Some may argue that the Imperial Absolute System is obsolete, but that's
true of the entire English, Imperial / British systems, yet in the U.S.
we still use these obsolete systems for trade, commerce, and even engineering.
This all seems like nonsense to the rest of the world, but we have had
users in Europe ask to have bullet weight in grains rather
than grams simply because grains fits the purpose better than
grams. We haven't done that primarily because there's a Metric
Grain and it's different than the English Grain, and that's
just too much confusion.
Scaling units to the purpose they serve and to human scale
has it's advantages over an arbitrary decimal ratio. It's
that convenience that has given rise to units like the metric
grain, the metric tonne, and KPH among users of the Metric
system. It's also why the U.S. is keeping it's measurement
system, which has been obsolete since the 1960's. For us,
scaled for use units are more important than easy conversion.
Now that conversions can readily be done by inexpensive calculators
and computer software maybe easy conversions is an obsolete