Suppose a friend of mine throws a ball for me to catch.  My friend does work on the ball which is converted into the ball’s kinetic energy.  As the ball moves up and then down as it flies through the air, some of the kinetic energy is converted into gravitational potential energy on its way up and then reconverted from potential energy back into kinetic energy on its way back down.  As the ball hits my hands as I catch it, the ball pushes my hands back, doing work on my hands.  This entire process has allowed at least some of the energy generated by my friend as she threw the ball to be transported to me.  The key point here is that matter moved to produce that transfer of energy.

Now suppose instead of throwing a ball, my friend stands the same distance away from me and starts clapping.  She is doing work as she moves her arms back and forth and smacks her hands together. As she brings her hands together, it creates a set of compressions and rarefactions in the air.  As those pressure differences reach my ear, they cause my eardrum to vibrate, which also vibrates the bones in the middle ear.  Those vibrations are passed on to hair cells in the inner ear, which convert those vibrations into electrical signals which are sent to the brain.  Once again, my friend did work as she clapped and that energy was transported to me as evidenced by the fact that I heard her claps.  So, did matter also move with this energy transfer?

The answer is yes, matter moved, but not in the same way it did with the ball.  With the ball, all the matter that was used to transport the energy moved from my friend to me.  With the sound, the air molecules near my friend began to vibrate back and forth, which caused them to bump into neighboring molecules and start them vibrating as well.  This allowed the molecules of air to basically stay where they were (just bouncing back and forth off each other) while the energy was transported as kinetic energy of the molecules that was transferred during the collisions.  This process of transporting energy without transporting matter is called a wave.  Waves that need a medium to travel through, so that there is something that can move to transfer its energy to its neighboring particle, are called mechanical waves.  We will focus on two types of mechanical waves in this section, sound waves and waves on a string.