3
Waves on the Move
E
arthquakes result when rocks within the earth fail to withstand the
stresses that occur when plates push on each other. As pressure
builds, the strength of the rock is eventually overcome and the plates move
past each other. Sometimes plates move just a little. Other times, they move
dramatically. Earthquakes cause the ground we stand on to move horizontally, vertically, or some combination of both.
When an earthquake occurs, more than one kind of wave is transmitted
through the earth. The two basic types of seismic waves that are transmitted are caused by the compression (P-waves) and shearing (S-waves)
of the earth. Because of how these two kinds of wave transmit through
the medium, they travel at different speeds through the earth. In this
activity you will use slinkies like the ones below to model these two kinds
of seismic waves.
CHALLENGE
(OW CAN A SLINKY BE USED TO MEASURE THE SPEED OF DIFFERENT KINDS OF WAVES
MATERIALS
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9
!CTIVITY s 7AVES ON THE -OVE
PROCEDURE
Part A: Exploring Wave Characteristics
1. With one student holding each end of the slinky, carefully stretch it
along the floor to a length of about 3 meters.
2. Take turns with your partner and practice making shearing and compression waves.
3. Vary the amplitude, or its side-to-side displacement from its state of
rest, of the shearing wave. In your science notebook, draw and label two
contrasting diagrams that show waves with relatively large and small
amplitude.
4. Using a timer, measure and record how long it takes for shearing waves
of different amplitudes to travel to the other end and back.
5. Vary the wavelength, or the distance between any two successive
identical parts of the wave, of both kinds of waves. In your science
notebook, draw and label two contrasting diagrams that show a
relatively short and long wavelength for both waves.
6. Vary the frequency, or the rate the medium is displaced, of both kinds
of waves. In your science notebook, draw and label two contrasting
diagrams that show a relatively fast and slow frequency.
Part B: Measuring Wave Speed
7. Practice making a shearing wave with a consistent wavelength and
frequency.
8. Measure the wavelength and frequency of the consistent shearing wave.
Record your data in your science notebook.
9. Using the meter stick and timer, measure the time it takes for the
consistent shearing wave to travel from one end of the slinky to the other.
10. Repeat Steps 7-9 for a compression wave with the same frequency.
11. The wave speed, or the rate a wave transmits energy, can be calculated
using either of the two formulas below:
a. speed distance xtime
b. speed frequency wavelength
Using the data you collected, calculate and record the wave speed using
both formulas.
7AVES ON THE -OVE s !CTIVITY 12. Predict what you think will happen to the speed of the wave when Steps
7-11 are repeated with a higher frequency wave. Record your prediction
in your science notebook.
13. Repeat Steps 7-10 with a higher frequency wave. Record your results in
your science notebook.
ANALYSIS
1. 7HICH TRAVELED FASTER THE SHEARING OR THE COMPRESSION WAVE $ESCRIBE
THE EVIDENCE YOU USED TO DECIDE 7HY DO YOU THINK IT IS THIS WAY
2. Earthquakes are made up of primary compression waves (P-waves) and
secondary shearing waves (S-waves) that move out from the center of
the earthquake in all directions.
a. 7HICH ONES DO YOU THINK WILL HIT THE SURFACE OF THE EARTH FIRST
Explain.
b. 7HICH DO YOU THINK WILL BE MORE DESTRUCTIVE %XPLAIN
3. The speed of sound in air is about 330 meters per second (m/s). If the
frequency of the sound increases to produce a higher pitch, does the
WAVELENGTH GET LONGER OR SHORTER %XPLAIN USING THE FORMULA FOR SPEED
4. Look at the data below of a similar slinky lab. Use the data to make at
least two conclusions about wave speed.
Slinky Data
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