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Weight (or, better yet, mass)
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Measuring the mass of something is more difficult than that of length and time. We have a secondary standard based on an atomic scale, which uses the carbon-12 atom. This isotope of carbon is given a mass equal to 12 atomic mass units, where an atomic mass unit (1 u) is equal to 1.6605402 X 10-27kg. This number, however, is uncertain to 1 part in a million. Until a more accurate measurement of an atomic mass unit can be made, we rely on the standard kilogram as our unit of mass. This standard is based on a volume of pure water. One cubic centimeter of water is assigned a mass of 1 gram, and 1,000 cubic centimeters (a 10 cm x 10 cm x 10 cm cube of water) a mass of 1 kilogram. As you can imagine, such a standard would be difficult to handle, so an equivalent standard, in the form of a platinum- iridium cylinder, is defined as the standard kilogram. This standard is kept at the International Bureau of Weights and Measures near Paris, to which copies are compared and kept in countries around the world. The U.S. copy is kept in a vault at NIST in Boulder, Colorado.

How is mass different from weight? (Find out and let me know).