To demonstrate the propagation of P waves, have a helper hold one end of a stretched out slinky. Give the slinky a push along its axis; the spring will compress and dilate as the compressional or longitudinal wave travels along the slinky. The speed of the wave is related to the spring's resistance to being compressed (its incompressibility). Ask the students to concentrate on a particular section of the slinky (any section will do) so they can see the spring compress and dilate.
Demonstrating S waves is similar, except now you generate the wave by
moving one end of the slinky in an up-down fashion. If the students
concentrate on a particular portion of the slinky, they will see that that part
goes up and down as the wave passes perpendicular to it. This is a shear (or
transverse) wave. In this case the speed of the wave is related to the
spring's resistance to being sheared (its rigidity). You can also illustrate
the concept of shear deformation with the slinky in its original, unstretched
shape. Ordinarily it has the shape of a cylinder, or if viewed from the side,
a rectangle. Holding this cylinder vertically, with one hand on the top and
one on the bottom, move the top part horizontally. The individual coils of the
slinky will move progressively to the side, much like a deck of cards. Note
that from the side the shape is changed to a rhombus, although the area
(actually the volume of the cylinder) is unchanged. Shear deformation involves
a change of shape with no volume change. To illustrate the rigidity of the
slinky, let go of the sheared slinky; it snaps back into its original shape.
Since the slinky is really just a coiled wire, both P and S waves in the slinky occur by bending these coils. Thus, the incompressibility and rigidity of the slinky are directly related to the resistance of these coils being bent. This means that, unlike the solid materials of the Earth, P waves and S waves in a slinky will probably travel at similar speeds. You don't have to mention this for the purposes of the demonstration, though.
Last modified: March 18, 1996 (jsb)