### Sound - Exemplar Solutions 3

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER – 13
Sound

16. A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?
Ans: Frequency is number of oscillations per second.
Frequency = (10/20) oscillations per second = 0.5 oscillations/sec
Time period is reciprocal of frequency. T = 1/f = 2 s

17. We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?
Ans: If the sound produced by a vibrating body is in the audible range, the sound produced will be heard by us.
If the sound produced by a vibrating body is not in the audible range, we will not be able to hear the sound even though the body is vibrating.

18. Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.
Ans: If a stick is struck against a frying pan in vacuum, the frying pan will vibrate. We will not be able to hear the sound of vibration because sound cannot travel in vacuum. Sound requires a medium such as air, water, glass, etc. for its propagation.

19. Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.
Ans: No, the two astronauts floating close to each other in space will not able be talk to each other without using any special device since there is vacuum in space and sound cannot travel in vacuum.

20. List three sources of noise pollution in your locality.
Ans: Vehicle noise, bursting of crackers, loudspeakers. (Any other reasonable sources of noise pollution should be accepted)