# Some Important Practical Units

Contents

## For Length/Distance

### (I) Astronomical Unit

It is the mean distance of the earth from the sun.

1 AU = 1.496×10¹¹ m

### (II) Light Year

It is the distance travelled by light in vacuum in 1 year.

1 ly = 9.46×10¹⁵ m

### (III) Parallactic Second

It is the distance at which an arc of length 1 astronomical unit subtends an angle of 1 second of arc.

1 Par Sec = 3.084×10¹⁶ m = 3.26 ly

1 μm = 10⁻⁶ m

1 nm = 10⁻⁹ m

1 A⁰ = 10⁻¹⁰ m

### (VII) Fermi

This unit is used for measuring nuclear size.

1 Fm = 10⁻¹⁵ m

## For Mass

(I) Pound, 1 Pound = 0.4536 kg

(II) Slug, 1 Slug = 14.59 kg

(III) Quintal, 1 Q = 100 kg

(IV) Tonne, 1 Ton = 1000 kg

(V) Atomic Mass Unit :- it is defined as the 1/12 th of the mass of one ₆C¹² atom.

1 amu = 1.66×10⁻²⁷ kg

## For Time

(I) Solar Day:- It is the time taken by the earth to complete one rotation about its own axis with respect to Sun.

(II) Solar Year:- It is the time taken by the earth to complete one revolution around the sun in its orbit.

1 Solar Year = 365.25 Days = 366.25 sedrial days