PRINCIPLES OF NAVAL ENGINEERING 



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Table 7-1. — Units of measurement in Several Common Systems. 



at the rate of 1 foot per second per second when 

 acted on by a force of 1 pound. In other words, 

 1 slug equals 32 pounds, 2 slugs equals 64 pounds, 

 and so forth. By using the slug as the unit of 

 mass, the British engineering system sets up 

 consistent units of measurement in which unit 

 force acting upon unit mass produces unit ac- 

 celeration. Note, however, that this is still a 

 gravitational system rather than an absolute 

 system. 



By this time it should be obvious that the 

 relationships expressed in one system of mea- 

 surement do not necessarily hold when a dif- 

 ferent system is used. When using any particular 

 system, it is essential to understand the precise 

 meaning of all terms used in that system. This 

 is not always a simple matter, since there are 

 an enormous number of possible combinations 

 of units and in many cases the same word is 



used to express quite different ideas. Another 

 source of confusion is the way in which the 

 various systems of measurement are used. In 

 everyday life we use the British gravitational 

 FPS system. In scientific work we use one of 

 the metric systems. In engineering and other 

 technical fields we use a British system or a 

 metric system, depending upon the field in- 

 volved. The only way to avoid total confusion in 

 the use of measurement terms is to make sure 

 that you understand the precise meaning of each 

 term, as it relates to the particular system 

 being used. 



The units shown in table 7-1 are only a few 

 of the units that may be derived in each system. 

 For example, the unit of pressure shown for 

 both of the British gravitational systems is 

 pounds per square foot (lb/ft2, or psf). How- 

 ever, the unit pounds per square inch (lb/in2, 



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