Vibrations and Waves: Energy and Motion
Have you ever been to a party where the music is so loud, you can feel it? I don’t mean just the beat in your mind. Sometimes you can literally feel the vibrations of the music in the floor or other objects. People often like to blast their music from their cars on the highway, causing unpleasant buzzing in the vehicles nearby. But, how do we get sensations like these from music playing so far away? How can music travel from somebody’s speakers and cause such powerful vibrations in other objects?
To answer this question, let’s explore what we mean when we say the word vibration. What are vibrations? Where do we find them? In a guitar string? On a trampoline? What about a cymbal in a drum set? The ground in an earthquake? These are all examples of things that experience vibrations. In each case, there is an object beginning at a resting position, such as the cymbal on its stand, or the string on the guitar. The motion made by the object is in a back-and-forth nature over the resting position. Vibrations are oscillating motions around a fixed position.
When an object vibrates, it exhibits a motion that repeats itself over the same path in a periodic fashion. That is, the time it takes to complete one cycle of the vibration is always the same. Take a look at this system here. We have a mass being suspended from a spring. When the system is at rest, the mass just sits there in the air, stretching the spring to a certain extent because of its weight. But, if someone pulls the mass down a little and lets go, the mass springs upward, past its resting position, and then oscillates up and down at a certain frequency. If we plot its position over the course of time, we see that there is a very regular, periodic nature to its movement. This nature is best represented in the form of a wave.
A wave is a disturbance that travels through a medium from one place to another. Waves are formed by the vibration of the object or substance that carries the wave. You can make your own waves in a jump rope by creating a vibration at one end. Begin with the jump rope in its resting position, then move one end quickly up and down. You’ll create a wave that travels through the rope from one end to the other. If you do it right, the wave might even bounce off the other end and travel back to your hand.
All waves are caused by some type of vibration. Vibrations cause a disturbance in the medium that becomes the source of the wave. Think about water waves formed when you throw a rock into a pond. The rock hitting the surface causes the water to vibrate. This vibration disturbs the surrounding water, creating waves that move outward from the point of the rock’s impact.
If we look more closely at a typical wave, we can see the periodic nature in the shape of the wave itself. Let’s plot the wave as a function of time. We’ll put seconds along the horizontal axis, and the vertical axis will measure the wave’s vertical displacement. Just so we can talk about the wave more clearly, we’ll label it with letters at different points. The letters A and E show the low points of the wave, which we call troughs. The letters C and G show the crests of the wave. A full wavelength is defined as that portion of the wave between two successive crests, or between two troughs. By looking at how the crests and troughs line up with our horizontal axis, we can see that a full wavelength always takes the same amount of time. That’s why we say that waves have a periodic nature.
Amplitude and Energy
But what about how high and low the wave oscillates? Can we learn anything from the height of a wave’s crest, or the depth of its trough? The distance between the midline of a wave and its crest or trough is called the amplitude. If this was a wave traveling through water, the amplitude would measure the maximum displacement of the particles from their resting point. We may not always see a wave’s amplitude, especially if it’s a sound wave or a light wave. But amplitude tells us something very important. It’s a measure of how much energy the wave is carrying. As the particles in the medium move up and down with the wave, they transfer energy in the direction that the wave is moving. Let’s go back to that rock you threw into the pond. The waves moving outward from the rock’s point of impact will eventually reach the shore. They’re transferring energy from the place where you impacted the water, and carrying the energy onto the shore. So, waves transport energy. But do they also transport matter?
Imagine that a duck was sitting on the pond at the time you threw in the rock. Your water waves would flow outward toward the shore. But would they carry the duck in to shore as well? Of course not! Ducks bob up and down on the surface of waves. The same thing happens with the molecules of water in the waves. The water itself just bobs up and down, transferring energy between particles as the wave moves along. Waves don’t actually transport any matter, even though they do transport energy. The waves on your jump rope transport energy, but the particles in the jump rope itself stay put. Sound waves carry energy through the air, but the air itself doesn’t go anywhere. So, waves transport energy without transporting matter.
The best way to experience this concept is through sound. When that driver blasts his music right next to you at a stoplight, the energy of the sound rattles your car without transferring any material your way. The same is true for the music at that party. We now know that sound waves carry energy from the music source – the vibrating speakers – toward the people and objects around them. They transfer this energy to the floor, the walls, and the ears of all the partygoers. Like all waves, sound waves carry energy without carrying matter. So even though they transfer energy to other objects, they don’t transfer any amount of mass or particles.
Vibrations in objects and substances are oscillating motions that occur repeatedly over a fixed position. Vibrations are the origins of waves. When vibrations disturb a medium, the disturbance travels through the medium, from one location to another, in the form of a wave. Waves travel by crests and troughs in a periodic fashion. The portion of a wave between two crests or troughs is called a wavelength. A wave’s amplitude indicates how much energy is being transferred by the wave. While waves do transport energy from one place to another, keep in mind that they do so without transporting matter.
After watching this lesson, you should be ready to:
- Define vibration and wave
- Identify and describe the parts of a wave
- Explain why a wave is called periodic
- Understand what a wave transports