What are the primary & derived quantities ? Explain

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In order to do any measurements some quantites need to be defined by comparison to a standard while all other quantities can be derived from these standards. The former are called primary or fundamental quantities while the rest are called derived quantities.

For the metric system there are standards that were kept in Paris. The kilogram was a specific lump of aluminum kept at 1 atmosphere pressure and 20 degrees C. (STP) A kilogram is defined as a mass equal to the mass of this aluminum standard kilogram. It is awkward to define a primary unit by comparison to a specific object kept somewhere on Earth so later primary units are defined by means that can be reproduced by any laboratory on Earth. The meter is defined a 1/10000 of the Earth's circumference at the equator, a hard thing to measure as it turns out, so Paris also had an aluminum meter stick at STP for ease of comparison. Time is now defined by the period of a specific spectral line of Cesium. Charge is another metric primary unit.

Once you have defined you primary unit then all other units a derived from that. The primary units of the metric system are the kilogram, the meter, the second, and the Coulomb of charge. Derived units can be expressed in just the units. Listed below are some examples of metric units that are derived:

1 gee = 9.8 m/sec 2

1 Newton = 1 kg m/sec 2

1 Joule = 1 kg m 2 /sec 2

velocity is in m/s

momentum is in kg m /sec

impulse is in Newton sec = kg m / sec

G is in N m 2 /kg 2= m 3 /kg sec 2

k the Coulomb law constant = N m2 /C 2 = kg m 3/ C 2 sec 2 (C is for Coulomb, not Celsius)

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