By the end of this section, you will be able to do the following:
The learning objectives in this section will help your students master the following standards:
[BL][OL][AL] Review amplitude, period, and frequency for simple harmonic motion. In the chapter on motion in two dimensions, we defined the following variables to describe harmonic motion:
For waves, these variables have the same basic meaning. However, it is helpful to word the definitions in a more specific way that applies directly to waves:
In addition to amplitude, frequency, and period, their wavelength and wave velocity also characterize waves. The wavelength λ λ is the distance between adjacent identical parts of a wave, parallel to the direction of propagation. The wave velocity v w v w is the speed at which the disturbance moves.
Wave velocity is sometimes also called the propagation velocity or propagation speed because the disturbance propagates from one location to another. Consider the periodic water wave in Figure 13.7. Its wavelength is the distance from crest to crest or from trough to trough. The wavelength can also be thought of as the distance a wave has traveled after one complete cycle—or one period. The time for one complete upanddown motion is the simple water wave’s period T. In the figure, the wave itself moves to the right with a wave velocity vw. Its amplitude X is the distance between the resting position and the maximum displacement—either the crest or the trough—of the wave. It is important to note that this movement of the wave is actually the disturbance moving to the right, not the water itself; otherwise, the bird would move to the right. Instead, the seagull bobs up and down in place as waves pass underneath, traveling a total distance of 2X in one cycle. However, as mentioned in the text feature on surfing, actual ocean waves are more complex than this simplified example.
This video is a continuation of the video “Introduction to Waves” from the "Types of Waves" section. It discusses the properties of a periodic wave: amplitude, period, frequency, wavelength, and wave velocity.
The crest of a wave is sometimes also called the peak.
Watch Physics: Amplitude, Period, Frequency and Wavelength of Periodic Waves. This video introduces several concepts of sound; amplitude, period, frequency, and wavelength of periodic waves. If you are on a boat in the trough of a wave on the ocean, and the wave amplitude is 1\,\text{m}, what is the wave height from your position? Since wave frequency is the number of waves per second, and the period is essentially the number of seconds per wave, the relationship between frequency and period is or just as in the case of harmonic motion of an object. We can see from this relationship that a higher frequency means a shorter period. Recall that the unit for frequency is hertz (Hz), and that 1 Hz is one cycle—or one wave—per second. The speed of propagation vw is the distance the wave travels in a given time, which is one wavelength in a time of one period. In equation form, it is written as or From this relationship, we see that in a medium where vw is constant, the higher the frequency, the smaller the wavelength. See Figure 13.8.
[BL] For sound, a higher frequency corresponds to a higher pitch while a lower frequency corresponds to a lower pitch. Amplitude corresponds to the loudness of the sound. [BL][OL] Since sound at all frequencies has the same speed in air, a change in frequency means a change in wavelength. [Figure Support] The same speaker is capable of reproducing both high and lowfrequency sounds. However, high frequencies have shorter wavelengths and are hence best reproduced by a speaker with a small, hard, and tight cone (tweeter), whereas lower frequencies are best reproduced by a large and soft cone (woofer). These fundamental relationships hold true for all types of waves. As an example, for water waves, vw is the speed of a surface wave; for sound, vw is the speed of sound; and for visible light, vw is the speed of light. The amplitude X is completely independent of the speed of propagation vw and depends only on the amount of energy in the wave.
In this lab, you will take measurements to determine how the amplitude and the period of waves are affected by the transfer of energy from a cork dropped into the water. The cork initially has some potential energy when it is held above the water—the greater the height, the higher the potential energy. When it is dropped, such potential energy is converted to kinetic energy as the cork falls. When the cork hits the water, that energy travels through the water in waves.
Instructions
A cork is dropped into a pool of water creating waves. Does the wavelength depend upon the height above the water from which the cork is dropped?
Students can measure the bowl beforehand to help them make a better estimation of the wavelength.
Geologists rely heavily on physics to study earthquakes since earthquakes involve several types of wave disturbances, including disturbance of Earth’s surface and pressure disturbances under the surface. Surface earthquake waves are similar to surface waves on water. The waves under Earth’s surface have both longitudinal and transverse components. The longitudinal waves in an earthquake are called pressure waves (Pwaves) and the transverse waves are called shear waves (Swaves). These two types of waves propagate at different speeds, and the speed at which they travel depends on the rigidity of the medium through which they are traveling. During earthquakes, the speed of Pwaves in granite is significantly higher than the speed of Swaves. Both components of earthquakes travel more slowly in less rigid materials, such as sediments. Pwaves have speeds of 4 to 7 km/s, and Swaves have speeds of 2 to 5 km/s, but both are faster in more rigid materials. The Pwave gets progressively farther ahead of the Swave as they travel through Earth’s crust. For that reason, the time difference between the P and Swaves is used to determine the distance to their source, the epicenter of the earthquake. We know from seismic waves produced by earthquakes that parts of the interior of Earth are liquid. Shear or transverse waves cannot travel through a liquid and are not transmitted through Earth’s core. In contrast, compression or longitudinal waves can pass through a liquid and they do go through the core. All waves carry energy, and the energy of earthquake waves is easy to observe based on the amount of damage left behind after the ground has stopped moving. Earthquakes can shake whole cities to the ground, performing the work of thousands of wrecking balls. The amount of energy in a wave is related to its amplitude. Largeamplitude earthquakes produce large ground displacements and greater damage. As earthquake waves spread out, their amplitude decreases, so there is less damage the farther they get from the source.
What is the relationship between the propagation speed, frequency, and wavelength of the Swaves in an earthquake?
In this animation, watch how a string vibrates in slow motion by choosing the Slow Motion setting. Select the No End and Manual options, and wiggle the end of the string to make waves yourself. Then switch to the Oscillate setting to generate waves automatically. Adjust the frequency and the amplitude of the oscillations to see what happens. Then experiment with adjusting the damping and the tension.
Which of the settings—amplitude, frequency, damping, or tension—changes the amplitude of the wave as it propagates? What does it do to the amplitude?
Calculate the wave velocity of the ocean wave in the previous figure if the distance between wave crests is 10.0 m and the time for a seagull to bob up and down is 5.00 s.
The values for the wavelength (λ=10.0 m) (λ=10.0 m) and the period ( T=5.00s) ( T=5.00s) are given and we are asked to find v w v w Therefore, we can use v w = λ T v w = λ T to find the wave velocity.
Enter the known values into v w = λ T v w = λ T v w = 10.0 m 5.00 s =2.00 m/s. v w = 10.0 m 5.00 s =2.00 m/s.
This slow speed seems reasonable for an ocean wave. Note that in the figure, the wave moves to the right at this speed, which is different from the varying speed at which the seagull bobs up and down.
The woman in Figure 13.3 creates two waves every second by shaking the toy spring up and down. (a)What is the period of each wave? (b) If each wave travels 0.9 meters after one complete wave cycle, what is the velocity of wave propagation?
To find the period, we solve for T= 1 f T= 1 f , given the value of the frequency ( f=2 s −1 ). ( f=2 s −1 ).
Enter the known value into T= 1 f T= 1 f T= 1 2 s −1 =0.5 s. T= 1 2 s −1 =0.5 s.
Since one definition of wavelength is the distance a wave has traveled after one complete cycle—or one period—the values for the wavelength (λ=0.9 m) (λ=0.9 m) as well as the frequency are given. Therefore, we can use v w =fλ v w =fλ to find the wave velocity.
Enter the known values into v w =fλ v w =fλ v w =fλ=(2 s −1 )(0.9 m) = 1.8 m/s. v w =fλ=(2 s −1 )(0.9 m) = 1.8 m/s.
We could have also used the equation v w = λ T v w = λ T to solve for the wave velocity since we already know the value of the period ( T=0.5s) ( T=0.5s) from our calculation in part (a), and we would come up with the same answer. 7.
The frequency of a wave is 10 Hz. What is its period?
8.
What is the velocity of a wave whose wavelength is 2 m and whose frequency is 5 Hz?
Use these questions to assess students’ achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify such objective and direct them to the relevant content. 9.
What is the amplitude of a wave?
10.
What is meant by the wavelength of a wave?
11.
How can you mathematically express wave frequency in terms of wave period?
12.
When is the wavelength directly proportional to the period of a wave?
