a boy is running towards a wall with a constant speed of 30m/s.When he is @ a distance of 0.9km he blows a whistle.If the speed of sound in air is 330m/s,find the time interval his distance from the wall when he hears the echo?

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ans:

5 sec,0.75 km

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Here,
Speed of boy=30 m/s
Speed of sound=330 m/s
Distance of boy from the wall=0.9 km=900 m
Suppose the required time =x
Then,
The distance travelled by boy in 'x' sec=30 x
Therefore, the distance travelled by sound in reaching the wall and coming back to the running boy=900+(900-30x)
=1800-3x
The time interval between original sound and its echo,
x=(1800-30x)/330
x=5 sec
The required distance between boy and wall=900-30 x=900-150=750m=0.75 km

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