Page 663 - Air and Gas Drilling Manual

Appendix
A

Dimensions and Units,
Conversion Factors

The systems of dimensions and units used in mechanics are based on Newton’s
second law of motion, which is force equals mass mutiplied by acceleration, or

F = m a (A-1)

for consistent systems of units.
In the English unit system, engineers define a pound of force as the force
required to accelerate one slug of mass at the rate of one foot per second per second.
One slug of mass has a weight of approximately 32.2 lb when acted upon by the
acceleration of gravity present at the surface of the earth. Thus, Equation A-1 in the
English units is
2
1 lb = ( 1 slug) ( 1 ft/sec )

In the International System of Units (or SI metric), engineers define a newton of
force as the force required to accelerate one kilogram of mass at the rate of one meter
per second per second. Thus, Equation A-1 in the SI metric is
2
1 N = ( 1 kg) ( 1 m/sec )

The physicists, on the other hand, utilize a version of the SI metric that defines
a dyne of force as the force required to accelerate one gram of mass at the rate of one
centimeter per second per second.
Unfortunately, these different systems tend to create confusion. In many parts
of the world engineers use the kilogram for both force and mass units. With
universal adoption of metric SI, however, this confusion should gradually disappear.
Any system based on length (L), mass (M), and time (T) is absolute because it
is independent of the gravitational acceleration g. A system based on length (L),
weight, i.e., force (F), and time (T) is referred to as a gravitational system, since
weight depends on the value of g which in turn varies with location (i.e., altitude
and latitude). Hence the weight (W) of a certain mass varies with its location. This
variation is not generally considered in this text as the variation in the value of g is

A-1
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