Units

# Units - Units Most of the modern world uses the SI(Systeme...

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Units Most of the modern world uses the SI (Systeme Internationale, or metic) system of units in everyday and technical situations. The United States is the only country that does not. US Engineers must therefore be familiar with both SI and USCS (United States Customary System) units. More importantly, US Engineers must be aware of the fundamental difference between the two. SI units were developed with Newton’s 2 nd Law and Ampere’s Law in mind. As such, the fundamental dimensions are length (m), mass (kg), time (sec) and electrical charge (C). All other quantities have their units derived from these three and the appropriate physical law. For example, consider the units of force in the SI system. From Newton’s 2 nd Law: ° = ±² = ³´µ¶³ · ¸¹º » (1) By definition then the force unit in the SI system, the N, has dimensions of kg-m/sec 2 . Things are no so straightforward when working with USCS units because there are five fundamental dimensions: length, mass, time, force and electrical charge. These fundamental dimensions are: feet, sec, pounds-mass (lb m ), pounds-force (lb f ) and C. Two problems arise, one small and one large. The small problem is that the word “pound” is used for both mass and force so to distinguish between the two we add the modifiers “mass” and “force” The USCS dimension for mass is then lb m while that for force is lb f .

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