To determine which boy would hear a rock falling off a cliff one kilometer away, we need to consider the medium through which sound travels.
According to the table, the speed of sound is:
- In air: 344 m/s
- In water: 1,437 m/s
- In rock: 3,000 m/s
Sound travels fastest in solid materials (rock), followed by liquids (water), and slowest in gases (air).
Now, let's evaluate each option:
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A boy sitting in a tree with his ear to the bark - The boy is in the air but has direct contact with the bark (rock), so he would hear the sound very well due to the rock's higher speed of sound.
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A boy sitting on the grass - He is in the air, so he would hear the sound at 344 m/s, which is slower compared to the other options.
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A boy lying down with his ear to the ground - If he is lying on the ground, he would also have better contact with the rock (the ground), allowing him to hear the sound faster than someone simply sitting in the air.
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A boy swimming in the water - The speed of sound in water is significantly faster than in air (1,437 m/s), so he would be able to hear the rock falling relatively quickly.
Considering all the options, the boys that are likely to hear the rock falling the fastest would be the boy lying down with his ear to the ground and the boy swimming in the water. However, the boy lying on the ground might hear it more clearly due to the direct contact with the medium (rock).
Therefore, the best response would be a boy lying down with his ear to the ground, as he would likely hear the sound of the rock falling most effectively.