WAVES     WAVE MOTION Most information about our surroundings arrives as a wave: sounds are transpoted to our ears; light to our eyes and electromagnetic radiaiton to our mobile phones. Through wave motion, energy can be transferred from a source to a receiver without the transfer of matter between the two points.   A good visual example are the waves on the surface of water. When a stone is dropped into a lake, waves will be generate that travel outwards in expanding circles, with the centres as the source of the disturbance. The wave propagates, not the water.              Fig.1.   Snap shot of the waves on the surface of water.          At each point, the water bobs up an down. The shape of the wave          can be approximated by a sine curve.

 Fig. 2a. A harmonic wave: at any position , the disturbance        is a sinusoidal function of time .

 Fig. 2b. A harmonic wave: at any time , the disturbance        is a sinusoidal function of position .

 Fig. 3.   Wave or propagation velocity (phase velocity) .

 Exercise     Calculate the following parameters from the animation of a travelling wave:      amplitude     wavelength     period     phase velocity      frequency     angular velocity     wave number   Describe the motion of the particle (red) located at m.

 TRANSVERSE and COMPRESSIONAL (LONGITUDINAL) WAVES Many types of waves can be classified as transverse waves or longitudinal (compressional) waves.   Transverse wave – the particles of the medium vibrate up and down in a direction transverse (perpendicular) to the motion of the wave. Examples: waves on a stretched string, electromagnetic waves.          Fig. 4.   Transverse wave – the particle marked + moves up and down          executing simple harmonic motion. The wave advances 1 wavelength          in a time interval of 1 period.             Fig. 5.   Animation of a travelling transverse wave moving to the right.           Each particle executes SHM as they move up and down at right angles           to the propagation direction.     Longitudinal (compressional) wave – the vibration of the particles of the medium vibrate along the same direction as the wave is propagating. The wave is characterised by a series of alternate condensations (compressions) and rarefactions (expansions). The plots in figure 2 also represent a longitudinal wave - the wave function gives the displacement in the direction the wave is travelling. The compressions correspond to the crests and the rarefactions are the troughs. Example: sound waves in air.           Fig. 5.   Longitudinal wave – the particle marked + moves backward and           forward executing simple harmonic motion. The wave advances           1 wavelength in a time interval of 1 period. The particles oscillate over           very small distances, whereas the wave itself propagates over much larger           distances. The wavelength is the distance between adjacent compressions           or between adjacent rarefactions.      Motion along a slinky   Sound wave generated by a tuning fork   Sound wave travelling through the air

 Earthquakes Both transverse and longitudinal waves are produced when an earthquake occurs. S waves (shear waves)  ~ 5 km.s-1  –   transverse waves that travel through the body of the Earth. However, they can’t pass through the liquid core of the Earth. P waves (pressure waves)  ~ 9 km.s-1  –   longitudinal waves that travel through the body of the Earth. Only longitudinal waves can travel through a fluid, because any transverse motion would experience zero restoring force since a fluid is readily deformable. Since P waves are detected diametrically across the Earth, but not S waves, infers that the Earth’s core must be liquid. L waves (surface waves) – travel along the Earth’s surface. The motion is essentially elliptical (transverse + longitudinal). These waves are mainly responsible for the damage caused by earthquakes.

 Water waves A water wave is a surface wave that moves along the boundary between the water and the air. The motion of each water molecule at the surface is elliptical and so is a combination of transverse and longitudinal motions. Below the surface, the motion is only longitudinal.

 Tsunami Tsunami is the name given to the very long waves on the ocean generated by earthquakes or other events which suddenly displace a large volume of water. "Tsunami" is from "harbor wave" in Japanese. A tsunami is distinct from ordinary wind-driven ocean waves in that its source of energy is a water displacement event. The wave speeds for tsunamis are very high in deep water. The tsunami of December 26, 2004 travelled from near the island of Sumatra to the east coast of Africa in just over seven hours. It was initiated by an earthquake of magnitude 9 off the western coast of northern Sumatra.   The wave speed depends upon wavelength and the depth of the water for tsunamis at sea. As waves enter shallower water, their wavelength and wave speed diminishes, causing their amplitudes to greatly increase.   Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding 100 km in the deep ocean water, and by the time between these crests, ranging from 10 minutes to an hour. As they reach the shallow waters of the coast, the waves slow down and the water can pile up into a wall of destruction tens of meters or more in height. The effect can be amplified where a bay or harbour funnels the wave as it moves inland. Large tsunamis have been known to rise over 30 meters. Even a tsunami 3 - 6 meters high can be very destructive and cause many deaths and injuries.   Some tsunamis consist of a single crest while others develop a broad trough in advance of the main wave and a succession of smaller waves behind. It is the preceding trough, together with man's curiosity, that has been the cause of much loss of life. People attracted by the very low water as the tsunami approaches have gone out to walk on the newly exposed sea floor and have been drowned as the rising pulse flooded shoreward.          Depth of water (m)      10         50         200        2000        4000        7000        Velocity (km.h-1 )           40         80         160          500           700          950       Wavelength (km)           10         20           50          150            200        280       amazing numbers !!!

 Animation produced with wm_travelling.m   If you have any feedback, comments, suggestions or corrections please email: Ian Cooper   School of Physics   University of Sydney ian.cooper@sydney.edu.au