A ship sends a sound pulse straight down and the reflection returns after 4 seconds. Sound travels at 1500 metres per second in sea water. How deep is the water?
A6000 m
B3000 m
C1500 m
D375 m
Answer & Solution
Correct answer: B. 3000 m
1. The pulse travels down to the seabed and then back up to the ship.
2. Total distance equals speed multiplied by time, so 1500 metres per second times 4 seconds.
3. That gives a total travelled distance of 6000 metres.
4. The depth is only half of that path, because the pulse covered the depth twice.
5. Half of 6000 metres is 3000 metres, so the water is 3000 metres deep.
6. Choosing 6000 metres forgets that the trip is a there and back journey.
7. Choosing 375 metres divides by the speed instead of multiplying by it.
_Source: Siyavula Physical Sciences Grade 10 (Everything Science, CC BY 4.0), Chapter 10: Sound_
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