NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 3

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Question: 2.3

A calorie is a unit of heat or energy and it equals about 4.2 J where 1J = 1 kg Suppose we employ a system of units in which the unit of mass equals a kg, the unit of length equals the unit of time is Show that a calorie has a magnitude in terms of the new units.

Answer: 2.3

  • Given that,

  • New unit of mass

  • Hence, in terms of the new unit, 1 kg

  • In terms of the new unit of length,

  • And, in terms of the new unit of time,

Showing the unit of energy in image

Showing the Unit of Energy in Image

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Question: 2.4

Explain this statement clearly:

“To call a dimensional quantity ‘large’ or ‘small’ is meaningless without specifying a standard for comparison”. In view of this, reframe the following statements wherever necessary:

(a) Atoms are very small objects

(b) A jet plane moves with great speed

(c) The mass of Jupiter is very large

(d) The air inside this room contains a large number of molecules

(e) A proton is much more massive than an electron

(f) The speed of sound is much smaller than the speed of light.

Answer: 2.4

The given statement is true because a dimensionless quantity may be large or small in comparison to same standard reference. For example, the coefficient of friction is dimensionless. The coefficient of sliding friction is greater than the coefficient of rolling friction, but less than static friction.

(a) An atom is a very small object in comparison to a soccer ball.

(b) A jet plane moves with a speed greater than that of a bicycle.

(c) Mass of Jupiter is very large as compared to the mass of cricket ball.

Comparison of volume, speed and mass

Comparison of Volume, Speed and Mass

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(d) The air inside this room contains a large number of molecules as compared to that present in a geometry box.

(e) A proton is more massive than an electron.

(f) Speed of sound is less than the speed of light.