Skill 2.2 Identifying uncertainties in measurement

A large proportion of errors in research and engineering as well as in the classroom are due to treating units as if they were not part of the number. In all scientific disciplines, it is essential that a unit be associated with every value in every calculation unless the value is a dimensionless quantity. On September 

NASA s 125 million Mars Climate Orbiter was lost because of a data transfer where units were not included as part of the number. Most students first learn the importance of associating numbers with units in their high school chemistry class. 

SI is an abbreviation of the French Systeme International d Unites or the International System of Units. It is the most widely used system of units in the world and is the system used in science. 

The use of many SI units in the United States is increasing outside of science and technology. There are two types of SI units base units and derived units. The base units are  

Amperes and candelas are rarely used in chemistry. The name kilogram occurs for the SI base unit of mass for historical reasons. Derived units are formed from the kilogram, but appropriate decimal prefixes are attached to the word gram.  

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