Slinky Seismic Waves
To model the two main types of seismic waves that travel through the Earth during an earthquake using a Slinky.
Theory & Background
When an earthquake strikes, its energy races outward through the Earth as seismic waves. Two important kinds travel through the planet’s interior: P-waves (primary waves), which push and pull the ground back and forth in the same direction they travel, and S-waves (secondary waves), which shake the ground across the direction of travel.
P-waves move faster, so they always arrive first. The time gap between the fast P-wave and the slower S-wave is exactly how scientists figure out how far away an earthquake happened. A Slinky is the perfect tool to see both waves in action.
Required Materials
- A long metal or plastic Slinky spring
- A helper
- A smooth floor or long table
- A small ribbon or piece of tape
Estimated Time
About 20 to 30 minutes
Step-By-Step Procedure
Stretch the Slinky out along a smooth floor between you and a helper, each holding one end. Don’t stretch it so far that the coils flatten out.
Tie a small ribbon (or stick a piece of tape) to one coil in the middle so you can watch how that single spot moves.
Make a P-wave: quickly push your end of the Slinky straight toward your helper and pull it back. Watch a “squeeze” of bunched-up coils travel down the spring.
Watch the ribbon closely — notice it moves back and forth along the same line the wave is traveling, then returns to where it started.
Make an S-wave: with the Slinky stretched out, flick your end quickly to one side and back.
Watch the sideways hump travel down the Slinky, and see the ribbon move side to side, across the direction the wave is moving.
Make both kinds of waves a few times and compare how they look and how fast each one seems to travel.
Experiment Tips
- A longer, metal Slinky shows the waves much more clearly than a short plastic one. Keep your movements quick and small.
- To model the rolling surface waves that do the most earthquake damage, lay a rope on the floor and send a rolling wave down it.
Observation
The push-pull motion sends a compression — a bunched-up section of coils — racing down the Slinky, with the ribbon moving along the same line as the wave. The sideways flick sends a side-to-side wiggle down the Slinky, with the ribbon moving across the wave’s path.
Result & Conclusion
You’ve modeled the two waves seismographs detect. The compression wave is a P-wave (the coils move parallel to the wave) and the sideways wave is an S-wave (the coils move perpendicular to the wave). Because P-waves travel faster, they reach seismographs first — and the gap between the P-wave and the slower S-wave tells scientists exactly how far away the earthquake struck.