6.0 Important practical units

  Unit and Dimensions

A. For large distance (macro-cosm)

(a). Astronomical unit: It is the average distance of the center of the sun from the centre of the earth.
$$1\,A.U. = 1.496 \times {10^{11}}m$$

(b). Light year: It is the distance travelled by the light in vacuum in one year.
$$1\,ly = 9.46 \times {10^{15}}m$$

(c). Parsec: One parsec is the distance at which an arc $1\ A.U.$ long subtends an angle of one second.
$$1{\text{ parsec}} = 3.1 \times {10^{16}}m$$

B. For small distance (micro-cosm)

$1$ micron $=\ 10^{-6}\ m$
$1$ nanometer $=\ 10^{-9}\ m$
$1$ Angstrom $=\ 10^{-10}\ m$
$1$ fermi $=\ 10^{-15}\ m$

C. For small area

$1$ barn $=\ 10^{-28}\ m^2$

D. For heavy masses

$1$ quintal $=\ 100\ kg$
$1$ Ton $=\ 1000\ kg$
$1$ C.S.L. $=\ 1.4$ time the mass of the sun

E. For small masses

$1$ amu $=\ 1.67 \times 10^{-27}\ kg$

F. For small time

$1$ shake $=\ 10^{-8}\ s$

G. For large time

(a). Lunar month: It is the time taken by the moon to complete one rotation about its axis with respect to earth.

$1$ L.M. $=\ 27.3$ days

(b). Solar day: It is the time taken by the earth to complete one rotation about its axis with respect to sun.

(c). Sedrial day: It is the time taken by Earth to complete one rotation on its axis with respect to distant star.

H. For measuring pressure

$1$ bar $=\ 1$ atm pressure $=\ 10^5\ N/m^2\ =\ 760\ mm\ Hg$
$1$ Torr $=\ 1\ mm\ Hg$