U.S. Customary Units:The U.S. customary, or British system of units, also called the footpound-second (FPS) system, has been the common system in business and industry in English-speaking countries. Although this system will in time be replaced by SI units, for many more years engineers must be able to work with both SI units and FPS units, and both systems are used freely in Engineering Mechanics. As shown in the table, in the U.S. or FPS system, the units of feet (ft) for length, seconds (sec) for time, and pounds (lb) for force are selected as base units, and the slug for mass is derived from Eq. 1/1. Thus, force (lb) = mass (slugs) * acceleration (ft/sec2), or
slug = lb-sec2 / ft
Therefore, 1 slug is the mass which is given an acceleration of 1 ft/sec2 when acted on by a force of 1 lb. If W is the gravitational force or weight and g is the acceleration due to gravity, Eq. 1/1 gives
m (slugs)=W (lb)/ g (ft/sec2)
Note that seconds is abbreviated as s in SI units, and as sec in FPS units.
In U.S. units the pound is also used on occasion as a unit of mass, especially to specify thermal properties of liquids and gases. When distinction between the two units is necessary, the force unit is frequently written as lbf and the mass unit as lbm. In this book we use almost exclusively the force unit, which is written simply as lb. Other common units of force in the U.S. system are the kilopound (kip), which equals 1000 lb, and the ton, which equals 2000 lb.
The International System of Units (SI) is termed an absolute system because the measurement of the base quantity mass is independent of its environment. On the other hand, the U.S. system (FPS) is termed a gravitational system because its base quantity force is deﬁned as the gravitational attraction (weight) acting on a standard mass under speciﬁed conditions (sea level and 45 degree latitude). A standard pound is also the force required to give a one-pound mass an acceleration of 32.1740 ft/sec2.
In SI units the kilogram is used exclusively as a unit of mass—never force. In the MKS (meter, kilogram, second) gravitational system, which has been used for many years in non-English-speaking countries, the kilogram, like the pound, has been used both as a unit of force and as a unit of mass.