# 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

## 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?

Explanation:

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

Explanation: Mass spectrograph

22). Express unified atomic mass unit in kg.

Explanation:

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?

Explanation:

∴ 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.

Explanation:

But

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.

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.

Explanation:

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.

Explanation:

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.

Explanation:

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