If a tree falls in the forest, and no one is around to hear it, does it make a sound?
When a ball hits the ground or an axe hits a tree, we can hear a noise signaling this collision. Obviously, sound waves are produced, but where exactly do they come from?
When two objects collide, one of two things can happen: an elastic or inelastic collision. In the case of elastic, no kinetic energy is lost. Inelastic, however, involves a loss of kinetic energy. Where does it go?
Part of it goes to heat, but another part of it causes the sound waves to be produced because they need energy. When two objects collide, the molecules of the object vibrate a little, which in turn vibrates the air molecules, creating a longitudinal wave.
So, if a tree falls, it does make a sound because the laws of physics don't stop just because there isn't a human to watch it.