NCERT Physics Class 11 Exemplar Chapter 2 Units And Measurements Part-3

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18). Which of the following are not a unit of time?

(a) Second

(b) Parsec

(c) Year

(d) Light year

Answer: (b), (d)

Very Short Answer Type Questions

19). Why do we have different units for the same physical quantity?

Explanation: Because, bodies differ in order of magnitude significantly in respect to the same physical quantity. For example, interatomic distances are of the order of angstroms, inter-city distances are of the order of km, and interstellar distances are of the order of light year.

20). The radius of atom is of the order of 1 Å and radius of nucleus is of the order of fermi. How many magnitudes higher is the volume of atom as compared to the volume of nucleus?


21). Name the device used for measuring the mass of atoms and molecules.

Explanation: Mass spectrograph

22). Express unified atomic mass unit in kg.


23). A function is defined as:

Why is it necessary for q to be a dimensionless quantity?

Explanation: Since is a sum of different powers of θ, it has to be dimensionless

24). Why length, mass and time are chosen as base quantities in mechanics?

Explanation: Because all other quantities of mechanics can be expressed in terms of length, mass and time through simple relations.

Short Questions

25). (a) The earth-moon distance is about 60 earth radius. What will be the diameter of the earth (approximately in degrees) as seen from the moon?


∴ Diameter of the earth as seen from the moon is about 2°.

(b) Moon is seen to be of (½)°diameter from the earth. What must be the relative size compared to the earth?

Explanation: At earth-moon distance, moon is seen as diameter and earth is seen as 2° diameter. Hence, diameter of earth is 4 times the diameter of moon.

(c) From parallax measurement, the sun is found to be at a distance of about 400 times the earth-moon distance. Estimate the ratio of sun-earth diameters.



26). Which of the following time measuring devices is most precise?

(a) A wall clock

(b) A stop watch.

(c) A digital watch.

(d) An atomic clock.

Give reason for your answer.

Explanation: An atomic clock is the most precise time measuring device because atomic oscillations are repeated with a precision of 1s in 1013 s.

27). The distance of a galaxy is of the order of 1025 m. Calculate the order of magnitude of time taken by light to reach us from the galaxy.


28). The Vernier scale of a travelling microscope has 50 divisions which coincide with 49 main scale divisions. If each main scale division is 0.5 mm, calculate the minimum inaccuracy in the measurement of distance.


29). During a total solar eclipse the moon almost entirely covers the sphere of the sun. Write the relation between the distances and sizes of the sun and moon.