The Correct Calculation !

Classical Mechanics Level 1

In an experiment to measure the speed of sound in air, a boy stands 40 m from a wall and bangs two pieces of wood together. At the instant he hears the echo, he bangs them together again. He does this activity 50 times. The time taken for 50 bangs is 12 s. Which calculation gives the speed of sound in air ?

(2 12 50)/40 (40*50)/12 (40 2 50)/12 (40 2 12)/50

Tushar Malik by Tushar Malik , 20, India

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6 solutions

Christo George
Aug 15, 2014

An echo is the reflection of sound wave(min dist for echo is 17mts). So the distance traveled by the wave should be multipled by 2.

Total no of reflection(sound waves produced) is 50.

Total time taken is 12s.

Since speed=dist÷time,

speed=(40×2×50)÷12

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Fel Arth Llena
Aug 15, 2014

40*2 it is because there are two pieces of wood then multiply with 50it beause theres a word 50 timesthen divide it by 12 it is because the time consumes by 50 bangs is 12

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2 is not the no of pieces of wood, it shows that the sound is reflected back to ears.

Christo George - 6 years, 10 months ago
K T
Nov 15, 2018

I think it is not a very precise experiment to perform this way. The reaction time and clapping motion of the boy will take hundreds of milliseconds per time, which is comparable to (if not larger than) the travel time of the sound over 80 m.

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Jaswanth Varma
Sep 21, 2014

It can be done logically as follows. We know that distance = speed * time. So speed = distance / time. Here dsitance = 40 * 2 * 50. (Since 40m in to direction and 40m again in the fro direction. And he banged it 50 times. So it is multiplied by 50.) Here time = 12 sec. So the answer would be (40 * 2 * 50) / 12.

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Cesario Pagador
Sep 12, 2014

40mtrs towards the wall and 40mtrs back to the boy's ears at 50 repetition that covers 12 seconds to complete. or 4000mtrs/12 secs or 333.333/sec or 19,999.999mtrs per hour makes the speed of sound

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Sam Brown
Aug 16, 2014

We know speed = distance/time.

Distance = (40 m (to the wall) + 40 m (back from the wall)) x 50 times

= 40 x 2 x 50

Time = 12 seconds in total.

Therefore, speed (of sound) = (40 x 2 x 50)/12

= 333.3 m/s if you must know.

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