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

Conversion table for length

  • Radius of hydrogen atom,

  • Volume of hydrogen atom

  • 1 mole of hydrogen contains hydrogen atoms.

Volume of 1 mole of hydrogen atoms