# NCERT Class 11 Physics Chapter 1 Units and Measurements Short Answer Type Questions

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**Question 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?

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

(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.

**Answer:**

(a) As the distance between moon and earth is greater than radius of earth, then radius of earth can be treated as an arc.

According to the problem,

Distance between moon and earth

So, angle subtended at distance due to an arc of length/ is

Hence, angle subtended by diameter of the earth

(b)

According to the problem, moon is seen as diameter from earth and earth is seen as diameter from moon.

As is proportional to diameter,

Hence,

(c) From parallax measurement given that Sun is at a distance of about times the earth-moon distance, hence,

(Suppose, here stands for distance and D for diameter)

Sun and moon both appear to be of the same angular diameter as seen from the earth.

But

**Question 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.

**Answer:**

Option (d) is correct because a clock can measure time correctly up to one second.

A stop watch can measure time correctly up to a fraction of a second. A digital watch can measure time up to a fraction of second whereas an atomic clock is the most accurate timekeeper and is based on characteristic frequencies of radiation emitted by certain atoms having precision of about 1 second in years. So an atomic clock can measure time most precisely as precision of this clock is about 1 s in

**Question 27:**

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

**Answer:**

According to the problem, distance of the galaxy

Speed of

Hence, time taken by light to reach us from galaxy is

**Question 28:**

The vernier scale of a travelling microscope has divisions which coincide with main scale divisions. If each main scale division is , calculate the minimum inaccuracy in the measurement of distance.

**Answer:**

Diagram of Vernier calliper is shown below.

According to the problem, divisions of Vernier scale which coincide with main scale divisions.

Where MSD=Main scale division and VSD=Vernier scale division.

We know that

Minimum inaccuracy

It is given in the problem that

Hence, minimum inaccuracy