Tuesday, March 20, 2012

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...

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