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would lead in general to the appearance of several additional
numerical factors in the equations between the numerical
values. It is possible, however, and in practice more convenient,
to choose a system of units in such a way that the
equations between numerical values, including the numerical
factors, have exactly the same form as the corresponding
equations between the quantities. A unit system defined in this
way is called coherent with respect to the system of quantities
and equations in question. Equations between units of a
coherent unit system contain as numerical factors only the
number 1. In a coherent system the product or quotient of any
distinct units from mass and force. In SI, the name kilogram
is restricted to the unit of mass, and the kilogram-force (from
which the suffix force was in practice often erroneously
dropped) is not to be used. In its place the SI unit of force,
the newton is used. Likewise, the newton rather than the
kilogram-force is used to form derived units which include
force, for example, pressure or stress (N/m2 = Pa), energy
(N . m = J), and power (N . m/s = W).
2.2 Considerable confusion exists in the use of the term
weight as a quantity to mean either force or mass. In common
use, the term weight nearly always means mass; thus, when
one speaks of a person's weight, the quantity referred to is
mass. In science and technology, the term weight of a body
has usually meant the force that, if applied to the body, would
give it an acceleration equal to the local acceleration of free
fall. The adjective "local" in the phrase "local acceleration of
free fall" has usually meant a location on the surface of the
earth; in this context the "local acceleration of free fall" has
the symbol g (sometimes referred to as "acceleration of
gravity") with observed values of g differing by over 0.5 per
cent at various points on the earth's surface and decreasing as
distance from the earth is increased. Thus, because weight is
a force = mass x acceleration due to gravity, a person's
weight is conditional on his location, but mass is not. A person
with a mass of 70 kg might experience a force (weight) on
earth of 686 newtons (=: 155 Ibf) and a force (weight) of only
113 newtons (= 22 lbf) on the moon. Because of the dual use
of the term weight as a quantity, the term weight should be
avoided in technical practice except under circumstances in
which its meaning is completely clear. When the term is used,
it is important to know whether mass or force is intended and
to use SI units properly by using kilograms for mass or
newtons for force.
2.3 Gravity is involved in determining mass with a
balance or scale. When a standard mass is used to balance the
measured mass, the direct effect of gravity on the two masses
is cancelled, but the indirect effect through the buoyancy of
air or other fluid is generally not cancelled. In using a spring
scale, mass is measured indirectly, since the instrument
responds to the force of gravity. Such scales may be calibrated
in mass units if the variation in acceleration of gravity and
buoyancy corrections are not significant in their use.
3. Energy and torque
two unit quantities is thd unit of the resulting quantity. For 3.1 The vector product of force and moment arm is
example, in any coherent system, unit area results when unit widely designated by the unit newton metre. hi^ unit for
length length iiss dmivuildteipdl ibeyd ubnyi t utnimit e,l eanngdth u, nuitn ifto rvceel owchiteyn w uhneitn muansist bfoenr deinnegrg my,o wmheincth oisr atolsroq unee wretsounl ts in co~nff utosiroqnu ew iist hex tphree susneidt
is multiplied by unit acceleration. as newton metre uer radian. the relationshiu to energy is
clarified, since the'product of torque and anhlar rotati& is Note.- Figure B-I illustrates the relationship of the units energy:
of the SZ.
(N . m/rad) . rad = N . m
3.2 If vectors were shown, the distinction between energy
2. Mass, force and weight and torque would be obvious, since the orientation of force
and length is different in the two cases. It is important to
2.1 The principal departure of SI from the gravirnetric recognize this difference in using torque and energy, and the
system of metric engineering units is the use of explicitly joule should never be used for torque.
Attachment C
Table C-1 (cont.)
Annex 5 - Units of Measurement to be used in Air
and Ground Operations
To convert from to Multiply by
lbf .in
lbf in/in
lbf .s/ftz
Ibf/ft
lbf/ft2
lbf/in
lbf/inZ (psi)
lbf/lb (thrust/weight (mass) ratio)
　
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