The Physics Philes, lesson 93: Do the Wave
We’re chugging along in our journey through introductory physics. We’ve made it through a discussion of fluid mechanics. Now it’s time to start learning about mechanical waves.
Before we dive in, we need to nail down the basics.
A wave is a fairly familiar thing. Most of us have seen a wave at some point. In general, a wave occurs whenever a system is disturbed from equilibrium. When that disturbance travels from one region to another, we say that the wave propagates.
In this set of posts, we’re going to deal with mechanical waves. Mechanical waves are disturbances that travel through a substance or medium (as opposed to, say, electromagnetic waves, which can travel through a vacuum). As a wave of this type travels, the particles in the medium undergo various types of displacements.
The types of displacements have to do with the type of wave we’re dealing with. There are three types. The first is a transverse wave. Think about holding a jump rope and giving it a good shake up and down. The displacement of the medium is perpendicular – or transverse – to the direction the wave is traveling. The second type of wave is a longitudinal wave. Particles in a medium undergoing the displacement characteristic of a longitudinal wave are displaced from side to side. The displacement and pressure fluctuations travel down the length of the medium. The displacements of the particles are back and forth parallel to the direction the wave is traveling. The third type of wave is a combination of the two. The particles kind of move in a little circle. Even though these types of waves have their differences, the motion of all of these waves is a disturbance from the equilibrium state that travels throughout the medium.
There are three other things that all mechanical waves have in common. First, the disturbances propagate with a definite speed through the medium. This is appropriately called the speed of propagation or wave speed and it isn’t described by just one number. The speed of propagation is determined by the mechanical properties of the medium. Second, the medium doesn’t travel. Check out this video starting at about 14:00 and ending around 14:50:
See the particles in the wave tank? They are moving around their equilibrium position, but not moving along with the wave. It’s the disturbance pattern that travels, not the medium itself.
Finally, to get these waves you have to put energy into the system by doing mechanical work. It’s important to remember that waves transport energy, not matter, from area to another.
So there you have it! The basics of mechanical waves. Now we’re ready to start a more in depth discussion next week with a look at periodic waves.