Seismic Waves – Wave Speed
Background:
Two different kinds of waves are used in seismic exploration. In a P-Wave (compressional wave), the
particles of Earth move back and forth in the direction the wave travels. In a S-Wave, the particles of Earth
move at right angles to the direction of the wave. In this activity we will discover how S and P waves are alike
and how they differ.
Materials:
Meter sticks
Slinky
Stopwatch
Procedure:
1. Work in groups of 2-3 students. Decide who will be the timer/data recorder in your group.
2. Make 2 data tables like the ones shown in the results section of this handout. One table will be for the
P-Wave (compressional wave) information and the second table will be for the S-Wave (transverse
wave) data. Be sure to include a title, use a ruler, and include units!!
3. On the desk or floor, stretch the slinky to a length of ½ meter. Record this length in your data table.
4. Make a P-wave by compressing 4-5 wires the end of the spring and letting go. Practice doing this until
you can make a wave that travels down the entire length of the spring.
5. Record the time it takes for the wave to travel down the spring. Do this twice for 2 different times.
6. CAREFULLY extend the spring another ½ meter and repeat steps 4 and 5.(1m total)
7. CAREFULLY extend the spring another ½ meter and repeat steps 4 and 5.(1.5m total)
8. CAREFULLY extend the spring another ½ meter and repeat steps 4 and 5.(2m total)
9. Repeat the experiment using the slinky to create an S-wave. An S-Wave (transverse wave) can be
created by gently pulling one spring, toward the end of the spring, to one side and letting go. Practice a
couple of times first.
Results:
Table 1: P-Wave (Primary Wave)
Time 1 (sec)
Trial Length of Spring
(cm)
1
50
2
100
3
150
4
200
Table 2: S-Wave (Secondary Wave)
Time 1 (sec)
Trial Length of Spring
(cm)
1
50
2
100
3
150
4
200
Time 2 (sec)
Avg. Time (sec)
Avg. Speed
(cm/sec)
Time 2 (sec)
Avg. Time (sec)
Avg. Speed
(cm/sec)
Calculations:
For each table, show a sample calculation for speed. Be sure to include the equation, work (neatly),
and circle the answer with the correct units. Don’t forget to add this to your table!
Speed = Distance/Time
Graph:
Make a line graph showing length vs. speed. Again, make sure you use a ruler, include a title,
and label axes with units. Don’t forget a key. You can plot both wave types on one set of axes.
Questions:
1. Draw a picture modeling an S-Wave and a P-wave. Write the type of wave below your diagram.
2. Discuss the relationship between length and speed for each wave type. (HINT: Look at the graph.)
3. Which wave type travels faster?
4. All waves lose energy as they travel. Wave types that lose less energy go farther. Which of the two
types of waves used in the lab needed less energy to travel?
5. Which wave probably goes deeper into the Earth’s layers? Why?
6. Why is it important to understand seismic waves?
7. How might this model not be a very good representation of a real seismic wave?
8. Would your data change if you did this lab on the carpet in the hallway?
9. What are 2 steps you took to reduce timing errors? Do you think they worked?
10. How does the tension in the spring relate to the travel of a real seismic wave?
(HINT: Think about the spacing of the coils in the slinky.)