sgi physics waves sb · the individual molecules or particles in the medium. when you observed the...

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CHALLENGE

The first wave to reach the surface in an earthquake is the primary wave (P-wave), which is a type of compression or longitudinal wave.

A longitudinal wave is one that transfers energy through compressions and rarefactions in the medium through which the wave travels. A sound wave is an example of a longitudinal wave. The secondary wave (S-wave), which is a type of shearing or transverse wave, reaches the surface after the P-wave. A transverse wave is one that causes the medium (or in case of electromagnetic waves, the electric field) to move perpendicular to the direction of propagation.

What characteristics do all waves have in common?

MATERIALS

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 04.01aLegacySansMedium 10/11.5

longitudinal wave

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 04.01bLegacySansMedium 10/11.5

transverse wave

P-waves are an example of a longitudinal wave (top). S-waves are an example of a transverse wave (bottom).

Chart continued on the following page

READINGComplete Student Sheet 4.1, “Anticipation Guide: Wave Characteristics,” to help prepare for the following reading.

Wave Energy Is EverywhereThe transfer of energy by waves can be observed from natural and human sources. Waves can carry small amounts of energy, such as the waves you observed in a slinky on the floor, sound waves, or water waves. Waves can also transfer tremendous amounts of energy, such as the energy transferred in earthquakes, tsunami waves, or gamma waves. The table below summa-rizes the characteristics associated with various types of waves.

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 03.01 no labelLegacySansMedium 10/11.5

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 03.01 no labelLegacySansMedium 10/11.5

Types of Waves

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 04.02LegacySansMedium 10/11.5

wave


water wave

shore

Transmission of Waves through Different MediaRegardless of the kind of wave or the amount of energy transmitted, it is important to note that when waves are transmitted through a medium; the medium itself is not transferred. In all cases, the energy is transmitted, not the individual molecules or particles in the medium. When you observed the piece of tape on the slinky in Activity 2, “Transmitting Wave Energy,” the tape moved back and forth but was not transmitted to the end of the slinky with the energy. Similarly, a boat will bob on a lake as the water waves transmit energy underneath it.

Mechanical waves such as earthquakes, sound waves, or waves in a slinky will travel differently depending on the medium. The same wave will travel at different speeds through two different substances. In general, waves travel faster through materials that are denser and have “springier” molecules. So sound generally moves faster through solids than liquids and faster through liquids than gases. For example, sound travels about fifteen times faster through metal than through air.

Some waves, such as light, are not mechanical and do not require a medium to be transmitted. For example, light can travel through the vacuum of outer space, whereas sound cannot. Another major difference between sound and light is that light travels much faster—at a rate of about 300,000,000 m/s compared to about 340 m/s for sound.

Fundamental CharacteristicsEvery wave has four fundamental characteristics: wavelength, frequency, amplitude, and speed. A wave’s wavelength is the length of one complete cycle, or the distance between any two successive identical parts of the wave. As you learned in the previous activity, the frequency is the rate the medium is displaced, or the number of full cycles of the wave per unit time. In sound waves, the frequency is a measurement of the pitch of the sound. A higher frequency sound has a higher pitch than a lower one. The amplitude is the wave’s displacement from its state of rest. In sound waves, the amplitude is a measurement of the loudness of the sound. A larger amplitude sound wave is louder than a similar sound wave with smaller amplitude. A diagram of wavelength and amplitude is shown below.


wavelength

amplitude

The speed of a wave is the distance it travels per unit time. Another way to determine a wave’s speed is by its frequency and wavelength using the formula,

speed (s) frequency (f) wavelength (L)

s f L

For example, a tsunami traveling across the deep ocean could have a wavelength of 400 km (400,000 m) and a frequency of 1 wave every 40 min (2400 sec). The speed would be,

s 1⁄2400 sec 400,000 m

167 m/s (374 miles per hour)

This means tsunamis can quickly travel across the ocean to coastal com-munities. Additionally, tsunamis typically have an amplitude of less than one meter when traveling in the deep ocean water. This can pose a danger because they may not be noticed until they are close to shore, when the tsu-nami reaches shallower water and its amplitude grows tremendously.

For a wave at constant speed, like the speed of light, the frequency and wavelength are inversely proportional. If the frequency decreases then the wavelength increases and vice versa. The diagram below shows this rela-tionship for a wave of constant speed.

How Earthquakes MoveIn an earthquake, longitudinal P-waves move faster than the transverse S-waves. For example, it could take 9 seconds for a P-wave to arrive but 64 seconds for the S-wave arrival, for a difference of 55 seconds. By timing how long it takes for both waves to arrive at the surface, scientists can determine the distance to the epicenter, or the point on the surface directly above the origin of the earthquake. Measurements from three seismic stations are used to find the location of the epicenter of an earthquake. The location of the epicenter is usually the site of the greatest damage. In the example on the next page, the seismic graphs on the left show the time interval between the arrival of the P-waves and the S-waves. Using this time interval and the wave speed, the distance from the epicenter for each seismometer is calcu-lated and plotted on a map.


short wavelengthhigh frequency

long wavelengthlow frequency

The fact that waves travel through media at different speeds has helped seismologists understand the earth’s composition. For example, by timing earthquakes carefully, scientists have discovered that waves slow down as they travel through the mantle of the earth. This is strong evidence that the core of the earth is denser than the crust of the earth. Similarly, the knowl-edge that S-waves cannot travel through liquids has helped determine that the outer core of the Earth is composed of liquid iron and nickel. In this way, seismic waves have provided a window into the earth’s interior.

ANALYSIS 1. How are seismic waves

a.

b.

2. Look at the surface waves diagram in the “Types of Waves” chart. Examine the motion of molecules in a water wave. Does this kind of

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 04.06aLegacySansMedium 10/11.5

seismometer 3

seismometer 2seismometer 1

epicenter

60 km

105 km

150 km

3158 SEPUP SGI Waves SEFigure: SGI Wv SE 04.06bLegacySansMedium 10/11.5

Dis

tanc

e (k

m)

60

200

150

105

7sec

17sec

12sec

22sec

seismograph 1

seismograph 2

seismograph 3

P-wave arrival

S-wave arrival

0 20 40 60Arrival time (seconds)

Arr

ival

tim

e in

terv

al S

-P

3. Look at the following pictures of different kinds of waves.

a.

b. For those characteristics that are not shared, explain how you know they are not.

amplitude frequency P-wavecompression medium speedepicenter seismic graphs wavelength

4. An earthquake with a frequency of about 50 cycles per second moves at 6000 m/second for the P-waves, and 3500 m/second for the S-waves. What is the wavelength of

a.

b.


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