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to the body, would give i t 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 (- I55 Ibf) and a force (weight) of only 113
newtons (= 22 Ibf) 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
3.1 The vector product of force and moment arm is
widely designated by the unit newton metre. This unit for
bending moment or torque results in confusion with the unit
for energy, which is also newton metre. If torque is
expressed as newton metre per radian, the relationship to
energy is clarified, since the product of torque and angular
rotation is energy:
(N . m/rad) . rad = N . m
3.2 If vectors were shown, the distinction between
energy and torque would be obvious, since the orientation of
force and length is different in the two cases. It is important
to recognize this difference in using torque and energy, and
the joule should never be used for torque.
Annex 5 - UnltJ of Measurement to be usmi in Air
and Ground Opemtions
Rase SI Units Derived SI Units with Special Names
joule
LENGTH
kilogram
MASS
TIME
FREQUENCY
AMOUNT OF
SUBSTANCE
ELECTRIC
CURRENT
THERMODYNAMIC
TEMPERATURE
TEMPERATURE
candela
LUMINOUS
INTENSITY ELECTRIC CONDUCTANCE
Supplementary Units
radian
PLANE ANGLE
ILLUMINANCE
SOLID ANGLE
Figure B-1.
21
Annex 5 --Units of M-rment t@.be usedin Air
. anii Gm& Openations Attachment B
4 . 4 1 prefixes
4.1. -Selection of prefixes
4.1.1 In general the S1 prefixes should be used to
indicate orders of magnitude, thus eliminating nonsignificant
digits and leading zeros in decimal fractions, and
providing a convenient alternative to the powers-of-ten
notation preferred in computation. For example:
12 300 mm becomes 12.3 m
12.3 x lo3 m becomes 12.3 km
0.00123 pA becomes 1.23 nA
4.1.2 When expressing a quantity by a numerical
value and a unit, prefixes should preferably be chosen so
that the numerical value lies between 0.1 and 1 000. To
minimize variety, it is recommended that prefixes
representing powers of 1000 be used. However, in the
following cases, deviation from the above may be indicated:
a) in expressing area and volume, the prefixes hecto,
deca, deci and centi may be required: for example,
square hectometre, cubic centimetre;
b) in tables of values of the same quantity, or in a
discussion of such values within a given context, it is
generally preferable to use the same unit multiple
throughout; and
c) for certain quantities in particular applications, one
particular multiple is customarily used. For example,
the hectopascal is used for altimeter settings and the
millimetre is used for linear dimensions in mechanical
engineering drawings even when the values lie outside
the range 0.1 to 1 000.
4.2. -Prefixes in compound units'
It is recommended that only one prefix be used in
forming a multiple of a compound unit. Normally the prefix
should be attached to a unit in the numerator. One exception
　
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