Chapter 2 Measurement of Physical Quantities


Chapter 2 Measurement of Physical Quantities

Measurement refers to the comparison of an unknown quantity with a standard. It judge against a quantity with a standard to see how many times as big as the standard the quantity is.

Background of Measurement

*Unit of the quantity = standard for describing magnitudes of physical quantities.

*1791 =metric system of measurement was established, Paris Academy of Sciences

o Meter was originally define as one ten-millionth of the distance from the equator to the North pole

o Second as the time for a pendulum one meter long to swing from one side to the other.

*1889 the definitions of the basic units have been established by an international organization, the general Conference on weights and Measures.

*1960 it has been known officially as the International System or SI (the abbriviation for the French equivalent, Syteme International)

o meter was define by an atomic standard, in terms of the wavelength of the orange- red light emitted by atoms of Krypton (86Kr) in a glow discharge tube; one meter was define as 1650763.73 of these wavelength.

o the unit of time was based on a certain fraction of the mean solar day, the average time interval between successive arrivals of the sun at its highest point in the sky.

*1967 is an atomic one, based on the two lowest energy states of the cesium atom.

o One second = the time required for 9192631770 cycles of this radiation

*November 1983 the standard was change again in a more radical way.

o Meter is the distance light travels in 1/299792458 second.

The three basic quantities are length, mass and time.

Length is a physical quantity for measuring space. The distance between two points.

Mass is a basic property of matter. The mass of an object is measure of the amount of matter it contains. The mass of an onject does not vary with the location. It remains the same even if the position of the object is changed with reference to the earth’s center.

Time is the interval between two successive events.

Length and its Units

Yard – In A.D. 1120, yard is measure from the standard length of the kings tip nose to the end of his outreached arm (King Louis XIV).

Meter was originally defined as one ten-millionth of the distance from the equator to the North Pole (1791).

Meter was defined by an atomic standard, in terms of the wavelength of the orange- red light emitted by atoms of Krypton (86Kr) in a glow discharge tube; one meter was defined as 1650763.73 of these wavelengths (1960).

Meter was redefined as the distance traveled by light in vacuum during a time of 1/ 299792458 second.

Mass and its Units

The MKS unit of mass is Kilogram. Kilogram (Kg) is defined as the mass of a specific platinum-iridium alloy cylinder kept at the International Bureau of Weights and Measures at Serves, France (1887).

Time and its Units

The unit of time was based on a certain fraction of the mean solar day, the average time interval between successive arrivals of the sun at its highest point in the sky (1960).


In 1967, the second/s is redefined as 9192631770 times the period of vibration of radiation from the cesium atom (Period is the time interval needed for one complete vibration).

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