# NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 9

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**Question: 2.15**

A famous relation in physics relates ‘moving mass’ *m* to the ‘rest mass’ of a particle in terms of its speed *v* and the speed of light, *c*. (this relation first arose as a consequence of special relativity due to Albert Einstein). A boy recalls the relation almost correctly but forgets where to put the constant c. he writes:

**Answer: 2.15**

Given the relation,

Dimension of

Dimension of

Dimension of

Dimension of

Dimension of

The given formula will be dimensionally correct only when the dimension of L.H.S is the same as that of R.H.S.

This is only possible when the factor, is dimensionless i.e. is dimensionless. This is only possible if is divided by Hence, the correct relation is

**Question: 2.16**

The unit of length convenient on the atomic scale is known as an angstrom and is denoted by the size of a hydrogen atom is what is the total atomic volume in of a mole of hydrogen atoms?

**Answer: 2.16**

Conversion of units of Base Quantities

Radius of hydrogen atom,

Volume of hydrogen atom

1 mole of hydrogen contains hydrogen atoms.

Volume of 1 mole of hydrogen atoms