# NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 9 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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