The direction of motion of the molecules of water is up and down, whereas the direction of travel of the resulting wave is to the right. A simple mechanism for generating and observing transverse waves in water is shown in Figure 2.3. As the wheel W rotates at constant speed the plunger Figure 2.3 Mechanism for producing and observing water waves. P moves up and down continuously with a type of motion that we shall discuss later in this chapter, called simple harmonic motion. Longitudinal Waves Longitudinal wave motion as well as transverse motion is important, and we shall see in the next chapter that sound is longitudinal wave motion in the air.
Longitudinal waves are more difficult to show in a sketch than trans verse waves are.
Each part of the spring moves very little from its equilibrium position. The displacement is in the direction of motion. If the end of the spring is moved with simple harmonic motion in a direction parallel to the spring, a harmonic longitudinal wave will be propagated along the spring. Longitudinal waves in air may be produced in a similar way if the spring is replaced by a long tube of air with a piston at the end moving with simple harmonic motion. This produces a sound wave.