|
|
S E C T I O N 16 . 1 • Propagation of a Disturbance 489 components. The transverse displacements seen in Figure 16.4 represent the variations The three-dimensional waves that travel out from points under the Earth’s surface along a fault at which an earthquake occurs are of both types—transverse and longitu- P waves (with “P” standing for pri- mary) because they travel faster than the transverse waves and arrive at a seismograph S waves (with “S” standing for secondary), travel through the Earth at 4 to 5 km/s near the surface. By recording the time interval between the arrivals of these two Consider a pulse traveling to the right on a long string, as shown in Figure 16.5. Figure 16.5a represents the shape and position of the pulse at time t ! 0. At this time, Active Figure 16.4 The motion of water elements on the surface of deep water in which a wave is propagating is a combination of transverse and longitudinal displace- ments, with the result that elements at the surface move in nearly circular paths. Each element is displaced both horizontally and vertically from its equilibrium position. Velocity of propagation A y (a) Pulse at t = 0 O vt x O y x v P (b) Pulse at time t P v Figure 16.5 A one-dimensional pulse traveling to the right with a speed v. (a) At t ! 0, the shape of the pulse is given by y ! f (x). (b) At some later time t, the shape remains unchanged and the vertical position of an element of the medium any point P is given by y ! f (x " vt). At the Active Figures link at http://www.pse6.com, you can observe the displacement of water elements at the surface of the moving waves. |