Q
TEACHING CLUES AND CUES
In what direction does the energy move? (It moves out from the
point at which the energy is released—the focus.)
Point out that in a P
wave the movement of
each coil of the Slinky is
parallel to the wave
Q
What happens within the body of the Earth when energy moves
through it in this way? (Energy is transmitted from any material
near the focus to particles away from the source in the form of
waves.)
C. Conclusion
Challenge pairs of students to decide what a P wave would look like
and draw one on the top half of a sheet of paper. Tell them to use
pencil, so if they change their minds later they can revise the drawing.
(There are several acceptable ways to do this.)
ADAPTATIONS AND EXTENSIONS
1. With students who show special interest, assign all of the reading
on Master 3.1a and add Rayleigh waves and Love waves to your
discussion.
2. Research and discuss (a) the intermolecular forces that provide the
means of transmitting energy in longitudinal waves and (b) the speed
of P waves traveling in the Earth’s crust. Ask:
motion (the length of the Slinky). In
an S wave, the movement is
perpendicular to the length of the
Slinky. This difference may be easier
for students to see than compression
and dilation.
Q
How does the density of materials affect the speed of P waves?
(The more dense the material, the faster the waves move.)
Q
As P waves travel deeper into the Earth, does their speed change?
(P waves, like S waves, move faster in the mantle than in the crust,
because the mantle is more dense. The rocks that compose the Earth
at great depths, however, are plastic, so they slow down the waves.)
Q
Why can P waves travel through liquids, while S waves cannot? (P
waves travel by compression, and S waves travel by shearing at
right angles to the direction of motion. Water cannot spring back
after it is sheared, so the S waves die out in water.)
Q
How are P waves similar to sound waves? (A sound wave in the
ground is a P wave; a P wave in the air is a sound wave. Both are
compressional waves.)
PART TWO
S
ECONDARY
,
OR
S
HEAR
W
AVES
(S W
AVES
)
MA T E R I A L S
Q
7.6 m (25 ft) of coiled telephone cord, available at electronics or
phone stores and some discount stores
Q
Two different colors of plastic tape, about 1 cm (.5 in.) wide
Q
Masking tape
Q
Two empty soda cans
A G U
/
F E M A
145
S
E I S M I C
S
L E U T H S
