Pressure and Shear Waves in Springs

In an experiment fitting the vernal equinox, I was mucking about with springs today. I've been having trouble getting waves to show up well on the high speed camera. The ubiquitous "slinky" behaves a little oddly. Perhaps I'll write about shear waves in slinkys at some point. So I set up a few general springs in hopes of getting a pressure wave (longitudinal wave motion) that travels faster than the shear wave (transverse wave motion).

First run, shot at 1000 fps.

• P-wave travel time: 30/1000ths second


• P-wave velocity: 953 cm/sec



• S-wave travel time: 34/1000ths second


• S-wave velocity: 841 cm/sec

A second run, this time shot at 3000 fps.

• P-wave travel time: 107/3000ths second


• P-wave velocity: 802 cm/sec



• S-wave travel time: 127/3000ths second


• S-wave velocity: 676 cm/sec

Both show the shear (transverse) wave moving at a slower velocity compared to the pressure (longitudinal) wave. One of the methods we have to estimate the location of earthquakes is due to the difference in travel times. Kind of like using the number of seconds between the thunder and the lighting flash to estimate how far away the storm is. The large quake that struck Mexico today was probably located in part by the difference in P- and S-wave travel times.

Now I believe this demo is ready for my intro students...

Thurs-Demo: The one with a Slinky

Today's demo: P and S-wave propagation using a slinky. When talking about earthquakes, it's often hard for students to "see" the relative motion of energy through rocks. We often use a slinky or a large spring to demonstrate the behavior of waves moving through a solid material. Trouble is, the movement of the wave is so fast, it's sometimes hard to see. What better way to slow things down than with the high-speed camera?

I haven't done any motion tracking, but I don't think the pressure wave is traveling any faster than the transverse (shear) wave in the spring - I think the spring is too "springy" so there's no discernible difference in velocity between the two.