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.
Generally, sound travels fastest in solids, followed by liquids, and slowest in gases. Based on the typical speeds of sound:
- In air (gas): Approx. 343 meters per second
- In water (liquid): Approx. 1482 meters per second
- In solids (like tree bark or the ground): can vary, but can be around 5000 meters per second or higher.
Given these speeds:
- A boy sitting on the grass (air): Sound will take longer to travel through the air.
- A boy swimming in the water (water): Sound will travel faster than in air, but still slower than through solids.
- A boy sitting in a tree with his ear to the bark (tree bark): The sound will travel quickly through the solid material.
- A boy lying down with his ear to the ground (ground): The sound will travel very quickly through the solid ground.
Considering the above, the boys with the best chances of hearing the rock falling quickly would be:
- A boy sitting in a tree with his ear to the bark - sound travels quickly through the bark.
- A boy lying down with his ear to the ground - sound also travels quickly through the ground.
Therefore, either the boy sitting in the tree or the boy lying down on the ground would likely be able to hear the rock falling off a cliff one kilometer away.
However, since both options seem equally viable based on the assumption that the tree bark effectively conducts sound well, any of these two boys would be able to hear the falling rock faster than the others.
If you have to choose one, I would select a boy lying down with his ear to the ground, as the ground typically provides better conduction of sound than the bark of the tree.
So, the response is a boy lying down with his ear to the ground.