why does an underwater swimmer hears an approaching motorboat before a swimmer on the surface does?
Sound moves faster in water than in air.
An underwater swimmer may hear an approaching motorboat before a swimmer on the surface due to the difference in the medium through which sound travels.
When sound waves move through a medium, such as air or water, they travel at different speeds. Sound travels approximately four times faster in water than in air. So, when a motorboat approaches, it creates sound waves that travel through both air and water.
When the sound waves reach the underwater swimmer, they travel more quickly through the water and reach their ears before the waves traveling through the air reach the surface and the swimmer on the surface. This delay is because the speed of sound is faster underwater compared to in air.
To understand this concept, you can try a simple experiment. Stand near a swimming pool and have someone submerge themselves underwater while making a noise, like tapping two objects together. You will observe that you hear the sound first in the air (at the surface) and later in the water. This illustrates the speed difference at which sound waves travel through each medium.
In summary, the underwater swimmer hears an approaching motorboat before a swimmer on the surface because sound travels faster in water than in air.