Other units of Length and Mass
Other units of Length:

  • Planck Length : Planck length is the fundamental quantum of length equal to 10-35 m. It is the distance travelled by light in a time interval of one Planck time. The smallest interval of time possible is 10 -43 s called the Planck time.
  • Fermi (F): One fermi or femtometre is equal 10-15 m. This unit is used to measure nuclear dimensions.
  • Angstrom (A): One Angstrom is equal to 10-10 m, used to measure atomic dimensions.
  • Astronomical Unit (A.U.): Used in astronomy to measure large distances connected with solar studies. One Astronomical unit is the mean distance between the sun and the earth.
    • 1 AU = 1.496 ×1011 m
  • Light Year (ly): The distance travelled by light in vacuum in one year is called one light year.

Distance = velocity × time One light year
One light year = 3 × 108 (m/s) × 1 year
= 3 × 108 × 365 × 24 × 60 × 60 × (m/s) × s
1 ly
 = 9.46 × 1015 m

This unit is used to measure intergalactic distance as well as stellar distances.

  • Parallactic second (Par sec): Par sec is a unit of length and not time. It is defined as the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc. It is also defined as the distance at which one astronomical unit subtends ah angle of one second of arc.

1 AU = 1.496 × 1011m
1 ly = 9.46 x 1015m
1 par sec = 3.0857 x 1016m
1 F = 10-15m
1 A° = 10-10m
1 micron or 1 μm = 10-6m
Other Units of Mass:

  • Atomic mass unit (u): One atomic mass unit (amu or u) is used as the unit of mass in atomic Physics. By international agreement the mass of 6C12.One 'u' is defined as (1/12)th of the mass of6C12 atom.
    1 atomic mass unit = 1u = 1.66054 × 10-27
    Mass of our galaxy is about 2 x 1041 Kg; the mass of one electron is 9.1 x 10-31 Kg
    1 microgram = 1 μg = 10-9 Kg
    1 gram = 1 g = 10-3 Kg
    1 pound = 1 lb = 0.453592 Kg