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1-1 Physical Quantities, standards, and Units

The laws of physics are expressed in terms of many differnt quantities:

• mass, length, time, force, speed, density, resistence, temperature, luminous intensity, magnetic field strength, and many more.

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Each of these terms has a precise meaning, they form part of a commong language that others physicists and other scientists use to communicate with, For example:

• When a physicist uses a term such as “kinetic energy”, the others understand what is meant.
  There must be agreement about the units used to express their values, it is impossible for scientist to communicate their results withoug agreements.

  Another example

  If a measurement of length is quoted as 4.3 meters, it means that the measured length is 4.3 times as long as the value accepted for a standard length defined to be “one meter”.

Measurement standard

Length and time were once regarded as fundamental quantities with their individual established standards; the measurement standard for speed (=length/time) could then be derived in terms of those standards.