Page 19 - Aeronautical Engineer Data Book

Fundamental dimensions and units 9
Setting out calculations in this way can help
avoid confusion, particularly when they involve
large numbers and/or several sequential stages
of conversion.

2.3.1 Force or thrust
The USCS unit of force or thrust is the pound
force (lbf). Note that a pound is also ambigu­
ously used as a unit of mass (see Table 2.3).

Table 2.3 Force (F) or thrust
Unit lbf gf kgf N

1 pound 1 453.6 0.4536 4.448
thrust (lbf)
1 gram 2.205 1 0.001 9.807
force (gf) 2 10 –3 2 10 –3
1 kilogram- 2.205 1000 1 9.807
force (kgf)
1 newton (N) 0.2248 102.0 0.1020 1

Note: Strictly, all the units in the table except the
newton (N) represent weight equivalents of mass
and so depend on the ‘standard’ acceleration due
to gravity (g). The true SI unit of force is the
2
newton (N) which is equivalent to 1 kgm/s .
2.3.2 Weight
The true weight of a body is a measure of the
gravitational attraction of the earth on it. Since
this attraction is a force, the weight of a body
is correctly expressed in pounds force (lbf).
Mass is measured in pounds mass (lbm) or
simply (lb)
2
Force (lbf) = mass (lbm) 2 g (ft/s )
2
Or, in SI units: force (N) = mass (kg) 2 g (m/s )
1 ton (US) = 2000 lb = 907.2 kg
1 tonne (metric) = 1000 kg = 2205 lb
2.3.3 Density
Density is defined as mass per unit volume.
Table 2.4 shows the conversions between units.