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Class 9Class 10First YearSecond Year
$2.5 A hody of mass 0.3 \mathrm{~kg} is attached to a horizontal spring. If the valve of the spring constant is 5 \mathrm{~N} / \mathrm{m} find the time period of the body if it is given a small displacement.$

$Example 2.1 A sitar string vibrates at 400 \mathrm{~Hz} . What is the time period of this vibration?$

$iv. Astronauts in space need to communicate with each other by radio links because(a) sound waves travel very slowly in space(b) sound waves travel very fast in space(c) sound waves cannot travel in space(d) sound waves have low frequency in space$

$vi. For a normal person audible frequency range for sound wave lies between(a) 10 \mathrm{~Hz} and 10 \mathrm{kHz} (b) 20 \mathrm{~Hz} and 20 \mathrm{kHz} (c) 25 \mathrm{~Hz} and 25 \mathrm{kHz} (d) 30 \mathrm{~Hz} and 30 \mathrm{kHz}$

$11.17. Describe the importance of acoustic protection.$

$11.9. What do you mean by the term intensity level of the sound? Name and define the unit of intensity level of sound.$

$11.7. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 \mathrm{~s} . If the speed of ultrasound through seawater is 1531 \mathrm{~ms}^{-1} what is the distance of the seabed from the ship?Ans. (2618 m)$