Physical Quantities,Standards and Units

Physical Quantities,Standards and Units



Law of physics are expressed in terms of physical quantities such as time, force etc.Physical quanties are often divided into fundamental and derived quantities.Derived quantities are those whose definitions are based on other physical quantities,eg speed,area etc.Fundamental quantities are not defined in terms of other quantities,eg.length,mass and time.

Physical quantities in general be divided into two classes:
1.Scalar Quantities
A scalar quantity is one which has only magnitude.
eg:mass,length,time,volume etc.

2.Vector Quatities
A vector quantity has both magnitude and direction.
eg:velocity,momentum,force etc.

In 1960, the General Conference of Weights and Measures recommended that a metric system of measurements called the International System of Units,abbreviated as SI units,be used.

Unit of Length
The SI unit of lenght of metre(m).

Various other metric units used for measuring lenghs are related to the metre by either multiples or submultiples of 10.

1 kilometre (km) = 10^3 m.
1 centimetre(cm) = 10^-2m.
1 millimetre(mm) = 10^-3m.
1 micrometer = 10^-6 m.
1 nanometre (nm) = 10^-9 m.
1 angstroms (A^0)= 10^-10m.
1 femtometre (fm)= 10^-15m.

1 lightyear = 9.46 * 10^15 m.

Unit of Mass
The SI unit of mass is the kilogram(kg).

1 tonne(t) = 10^3 kg.
1 gram(g) = 10^-3 kg.
1 milligram = 10^-6 kg.

Unit of Time.

The SI unit of time is the second(s).