# JEE Main & Advanced Physics Mathematical Tools, Units & Dimensions Practical Units

Practical Units

Category : JEE Main & Advanced

(1) Length

(i) 1 fermi = 1 fm = 10-15 m

(ii) 1 X-ray unit = 1XU = 10-13 m

(iii) 1 angstrom = 1Å = 10-10 m = 10-8 cm = 10-7 mm = 0.1 $\mu$mm

(iv) 1 micron = $\mu$m = 10-6 m

(v) 1 astronomical unit = 1 A.U. = 1. 49 x 1011 m = 1.5 x 1011 m $\approx$ 108 km

(vi) 1 Light year = 1 ly = 9.46 x 1015 m

(vii) 1 Parsec = 1pc = 3.26 light year

(2) Mass

(i) Chandra Shekhar unit : 1 CSU = 1.4 times the mass of sun = 2.8 x 1030 kg

(ii) Metric tonne : 1 Metric tonne = 1000 kg

(iii) Quintal : 1 Quintal = 100 kg

(iv) Atomic mass unit (amu) :  amu = 1.67 x 10-27 kg

Mass of proton or neutron is of the order of 1 amu

(3) Time

(i) Year : It is the time taken by the Earth to complete 1 revolution around the Sun in its orbit.

(ii) Lunar month : It is the time taken by the Moon to complete 1 revolution around the Earth in its orbit.

1 L.M. = 27.3 days

(iii) Solar day : It is the time taken by Earth to complete one rotation about its axis with respect to Sun. Since this time varies from day to day, average solar day is calculated by taking average of the duration of all the days in a year and this is called Average Solar day.

1 Solar year = 365.25 average solar day

or average solar day $=\frac{1}{365.25}$ the part of solar year

(iv) Sedrial day : It is the time taken by earth to complete one rotation about its axis with respect to a distant star.

1 Solar year = 366.25 Sedrial day

= 365.25 average solar day

Thus 1 Sedrial day is less than 1 solar day.

(v) Shake : It is an obsolete and practical unit of time.

1 Shake = 10? 8 sec

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