Q24 of 44 Page 8

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


In mechanics we have to deal with all the physical changes happening around us like force, pressure, momentum which are not fundamental quantities.


They are obtained from some physical quantities which are fundamental quantities.


Length mass and time are chosen as base quantities due to following reasons:


1. These quantities cannot be derived from other quantities.


2. They are the simplest units.


3. All other physical quantities can be written in terms of these units.


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22

Express unified atomic mass unit in kg.

 23

. A function F(θ) is defined as

F (θ) = 1 – θ + - +


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


 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.


 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.