Q26 of 44 Page 9

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.


The correct answer is (d). An atomic clock is based on the characteristic frequencies of radiation emitted by atoms of certain elements which have a precision of about 1s in 3 x 105 years. However, a wall clock can measure correctly up to one second, a stop watch can measure correctly about a fraction of a second and digital clock can also measure correctly up to about a fraction of second.


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24

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

 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 ( �)° 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.


 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.