To determine which boy would be able to hear a rock falling off a cliff one kilometer away, we need to consider the velocity of sound in different materials and how sound travels through them.
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A boy lying down with his ear to the ground: The sound would travel through the ground (rock), where the velocity of sound is much higher (3,000 m/s). Therefore, he would hear the rock falling very quickly.
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A boy swimming in the water: The sound travels faster in water (1,437 m/s) than in air, but the rock is falling from above, and the sound needs to pass from air to water. However, he would still hear the sound relatively well.
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A boy sitting on the grass: In this scenario, the sound travels through air at a velocity of 344 m/s. This is the slowest option, and he would hear the rock falling, but later than the other options.
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A boy sitting in a tree with his ear to the bark: If his ear is against the bark of the tree, the sound would also travel through the wood, which generally has a higher speed than air but is less than that of rock.
Based on these analyses, all the boys would be able to hear the rock falling, but the boy lying down with his ear to the ground would hear it the fastest due to the high velocity of sound in rock.
Answer: A boy lying down with his ear to the ground.
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