# Dictionary Definition

poundal n : a unit of force equal to the force
that imparts an acceleration of 1 foot/sec/sec to a mass of 1
pound; equal to 0.1382 newtons [syn: pdl]

# User Contributed Dictionary

## English

### Noun

#### Derived terms

# Extensive Definition

The poundal is a non-SI unit
of force. It is a part of
the foot-pound-second
system of units, a coherent subsystem of English units introduced
in 1879, and one of several specialized subsystems of mechanical
units used as aids in calculations. It is defined as 1 lb·ft·s−2,
or in words, as the force necessary to accelerate a pound of mass
at 1 foot per second, per second. 1 pdl =
0.138 254 954 376 N exactly.

English units require re-scaling of either force
or mass to eliminate a numerical proportionality constant in the
equation F = ma. The poundal represents one choice, which is to
rescale units of force. Since a pound of force accelerates a pound
of mass at about 32 ft/s2 (the acceleration of gravity, g), we can
scale down the unit of force to compensate, giving us one that
accelerates 1 pound mass at 1 ft/s² (rather than at
32 ft/s²); and that is the poundal, which is approximately
pounds of force.

For example, a force of 1200 poundals is required
to accelerate a person of 150 pounds mass at 8 feet per second
squared:

- (150 lbm) × (8 ft/s²) = (1200 pdl)

The poundal-as-force, pound-as-mass system is
contrasted with an alternate system in which pounds are used as
force (pounds-force), and instead, the mass unit is rescaled by a
factor of 32. That is, one pound-force will accelerate one
pound-mass at 32 feet per second squared; we can scale up the unit
of mass to compensate, which will be accelerated by
1 ft/s² (rather than 32 ft/s²) given the
application of one pound force; this gives us a unit of mass called
the slug, which
is about 32 pounds mass. Using this system (slugs and
pounds-force), the above expression could be expressed as:

- (4.66 slug) × (8 ft/s²) = (37.3 lbf)

Note that slugs and poundals are never used in
the same system, since each exists to solve the same problem, so
that both should not be used together.

Rather than changing either force or mass units,
one may choose to express acceleration in units of the acceleration
due to Earth's gravity (called g). In this case, we can keep
both pounds-mass and pounds-force, such that applying one pound
force to one pound mass accelerates it at one unit of acceleration
(g):

- (150 lbm) × (0.249 g) = (37.3 lbf)

The advantage of using poundals (rather than
using slugs or g) is that it is not tied to the conditions on the
surface of the Earth, since it is not based on Earth's gravity. One
pound-mass exerts a downward force of about one pound-force, but
only on Earth's surface; in space or on
the moon, for example, one
pound-mass does not exert a pound-force under natural gravity
conditions, and thus the pound-force becomes an arbitrary unit with
no meaningful properties anymore. The pound-mass, however, is the
same whether on Earth, in space, or anywhere else, and the
poundal— which accelerates it at one foot per second
squared— remains relevant.

poundal in Arabic: باوندال

poundal in German: Poundal

poundal in French: Poundal

poundal in Japanese: パウンダル

poundal in Turkish: Poundal