5 Waves 51-1 Bow waves Although we have finished our quantitative analyses of waves, this added 51-1 Bow wayes chapter on the subject is intended to give some appreciation, qualitatively, for rious phenomena that are associated with waves, which are too complicated to 51-2 Shock waves analyze in detail here. Since we have been dealing with waves for several chapters 51-3 Waves in solid more properly the subject might be called "some of the more complex phenomena 51-4 Surface waves The first topic to be discussed concerns the effects that are produced by source of waves which is moving faster than the wave velocity, or the phase velocity Let us first consider waves that have a definite velocity, like sound and light. If we have a source of sound which is moving faster than the speed of sound, then mething like this happens Suppose at a given moment a sound wave is generated from the source at point x in Fig. 51-1; then, in the next moment, as the source moves to x2, the wave from x1 expands by a radius ri smaller than the distance hat the source moves; and of course, another wave starts from x2. when the sound source has moved still farther, to xa, and a wave is starting there, the wave from x 2 has now expanded to r2, and the one from xI has expanded to ra.Of course the thing is done continuously, not in steps, and therefore, we have a series of wave circles with a common tangent line which goes through the center of the generating were standing still, it generates a wavefront which forms a cone in three dimensions a pair of lines in two dimensions. The angle of the cone is very easy to figure out. In a given amount of time the source moves a distance, say x3-xl, pro- portional to u, the velocity of the source. In the meantime the wavefront has moved out a distance ra, proportional to cu, the speed of the wave. Therefore it is clear that the half-angle of opening has a sine equal to the ratio of the speed of the waves, Fig. 51-1. The shock wave front lies divided by the speed of the source, and this sine has a solution only if cw is less on a cone with apex at the source and than v, or the speed of the object is faster than the speed of the wave v/cao. 51.1) Incidentally, although we implied that it is necessary to have a source of sound, it turns out, very interestingly, that once the object is moving faster than the speed of sound, it will make sound. That is, it is not necessary that it have a certain tone vibrational character. Any object moving through a medium faster than the speed at which the medium carries waves will generate waves on each side, auto- matically, just from the motion itself. This is simple in the case of sound, but it also occurs in the case of light. At first one might think nothing can move faster than the speed of light. However, light in glass has a phase velocity less than the a charged particle of very high energy through a block of glass such that the particle velocity is close to the speed of light in a vacuum, while the speed of light in the glass may be only 3 the speed of light in the vacuum. a particle moving faster than the speed of light in the medium will produce a conical wave of light with its apex at the source, like the wave wake from a boat (which is from the same effect. as a matter of fact). By measuring the cone angle, we can determine the speed of the particle. This is used technically to determine the speeds of particles as one of the methods of determining their energy in high-energy research. The direction of the light is all that needs to be measured 5I-1
Fig. 51-2. A shock wave induced in gas by a projectile moving faster the sound This light is sometimes called Cerenkov radiation, because it was first observed by Cerenkov. How intense this light should be was analyzed theoretically by Frank and Tamm. The 1958 Nobel Prize for physics was awarded jointly to all three for this work The corresponding circumstances in the case of sound are illustrated in Fig 51-2, which is a photograph of an object moving through a gas at a speed greater than the speed of sound. The changes in pressure produce a change in refractive index, and with a suitable optical system the edges of the waves can be made visible. We see that the object moving faster than the speed of sound does, indeed duce a conical wave. But closer inspection reveals that the surface is actually curved. It is straight asymptotically, but it is curved near the apex, and we have now to discuss how that can be, which brings us to the second topic of this chapter. t avefront"snapshotsat successive instants in time. 51-2 Shock waves ed often depe plitude, and in the case of sound the speed depends upon the amplitude in the following way. An object moving through the air has to move the air out of the way, so the disturbance produced in this case is some kind of a pressure step, with the pressure higher behind the wavefront than in the undisturbed region not yet reached by the wave(running along at the normal speed, say). But the air that is left behind, after the wavefront passes, has been compressed adiabatically, and therefore the temperature is increased. Now the speed of sound increases with the temperature, so the speed in the region behind the jump is faster than in the air in front. That means that any other disturbance that is made behind this step, say by a continuous pushing of the body, or any other disturbance, will ride faster than the front, the speed increasing with higher pressure. Figure 51-3 illustrates the situation, with some little bumps of pressure added to the pressure contour to aid visualization. We see that the higher pressi at the rear overtake the front as time goes on, until ultimately the compressional wave develops a sharp front. If the strength is very high, ""ultimately"means right away; if it is rather weak, it takes a long time; it may be, in fact, that the sound is spreading and dying out before it has time to do this The sounds we make in talking are extremely weak relative to the atmospheric pressure-only I part in a million or so. But for pressure changes of the order of 1 atmosphere, the wave velocity increases by about twenty percent, and the wavefront sharpens up at a correspondingly high rate. In nature nothing happens infinitely rapidly, presumably, and what we call a"sharp"front has, actually, a very slight thickness; it is not infinitely steep. The distances over which it is varying are of the 51-2
order of one mean free path, in which the theory of the wave equation begins to fail because we did not consider the structure of the gas Now, referring again to Fig 51-2, we see that the curvature can be understood if we appreciate that the pressures near the apex are higher than they are farther back, and so the angle e is greater. That is, the curve is the result of the fact that the peed depends upon the strength of the wave. Therefore the wave from an atomic bomb explosion travels much faster than the speed of sound for a while, until it gets so far out that it is weakened to such an extent from spreading that the pressure bump is small compared with atmospheric pressure. The speed of the bump then approaches the speed of sound in the gas into which it is going. (Incidentally,it always turns out that the speed of the shock is higher than the speed of sound in the gas ahead, but is lower than the speed of sound in the gas behind. That is mpulses from the back will arrive at the front but the front rides into the medium in which it is going faster than the normal speed of signals. So one cannot te acoustically, that the shock is coming until it is too late. The light from the bomb arrives first, but one cannot tell that the shock is coming until it arrives, because there is no sound signal coming ahead of it. This is a very interesting phenomenon, this piling up of waves, and the main point on which it depends is that after a wave is present, the speed of the resulti wave should be higher. Another example of the same phenomenon is the following Consider water flowing in a long channel with finite width and finite depth. If a piston,or a wall across the channel, is moved along the channel fast enough, water piles up, like snow before a snow plow. Now suppose the situation is as shown in Fig 51-4, with a sudden step in water height somewhere in the channel. It can be demonstrated that long waves in a channel travel faster in deeper water than they do in shallow water. Therefore any new bumps or irregularities in energy supplied by the piston run off forward and pile up at the front. Again, ultimately what we Figure 51-4 have is just water with a sharp front, theoretically. However, as Fig 51-4 shows there are complications. Pictured is a wave coming up a channel; the piston is at the far right end of the channel. At first it might have appeared like a well-behaved wave, as one might expect, but farther along the channel, it has become sharper and sharper until the events pictured occurred. There is a terrible churning at the surface, as the pieces of water fall down, but it is essentially a very sharp rise with no disturbance of the water ahead Actually water is much more complicated than sound. However, just to illus trate a point, we will try to analyze the speed of such a so-called bore in a channel The point here is not that this is of any basic importance for our purposes--it is not a great generalization--it is only to illustrate that the laws of mechanics that we already know are capable of explaining the phenomenon. Imagine, for a moment, that the water does look something like Fig. 51-5(a) that water at the higher height h2 is moving with a velocity v, and that the front is moving with velocity u into undisturbed water which is at height h1. We would like to determine the speed at which the front moves. In a time At a vertical plane initially at xt moves a distance v At to x2, while the front of the wave has moved Now we apply the equations of conservation of matter and momentum, First Fig. 51-5. Two cross sections of a he former: Per unit channel width we see that the amount h2 At of matter that bore in a channel, with(b)at an interval has moved past xI(shown shaded)is compensated by the other shaded region, Af later than (a) which amounts to(h2- h1)u Al. So, dividing by At, wh2 = u(h2-hi.That does not yet give us enough, because although we have h? and hl, we do not know either u or v; we are trying to get both of them Now the next step is to use conservation of momentum, We have not discussed the problems of water pressure, or anything in hydrodynamics, but it is clear any way that the pressure of water at a given depth is just enough to hold up the column of water above it. Therefore the pressure of water is equal to p, the density of water, times g, times the depth below the surface. Since the pressure increases linearly with depth, the average pressure over the plane at xI, say, is 2pgh2, which is also the average force per unit width and per unit height pushing the plane toward x2 So we multiply by another h2 to get the total force which is acting on the water
pushing from the left. On the other hand, there is pressure in the water on the right also, exerting an opposite force on the region in question, which is, by the same kind of analysis, ipghi. Now we must balance the forces against the rate of change of the momentum. Thus we have to figure out how much more momentu there is in situation(b)in Fig. 51-5 than there was in(a). We see that the additional ed v is just ph2u△t-ph2△t( multiplying this by v gives the additional momentum to be equated to the impulse ph2b△)=(lgh2-bpgh3) If we eliminate v from this equation by substituting uh2= u(h2 -h,), already found, and simplify, we get finally that u2=gh2h1 + h2)/2h1 If the height difference is very small, so that hi and h2 are nearly equal, this says that the velocity =vgh. As we will see later, that is only true provided the wavelength of the wave is longer than the depth of the channel We could also do the analogous thing for sound waves-including the con servation of internal energy, not the conservation of entropy, because the shock is irreversible. In fact, if one checks the conservation of energy in the bore problem, one finds that energy is not conserved. If the height difference is small, it is almost perfectly conserved, but as soon as the height difference becomes very appreciable there is a net loss of energy. This is manifested as the falling water and the churning wn in Fig. 51 In shock waves there is a corresponding apparent loss of energy, from the point of view of adiabatic reactions. The energy in the sound wave, behind the shock, goes into heating of the gas after shock passes, corresponding to churning of ater in the bore In working for the out to be necessary for solution, and the temperature behind the shock is not the same as the temperature in front, as we have try to make a bore that is upside down(h2 hi), then we find that the energy loss per second is negative. Since energy is not available from anywhere, that bore cannot then maintain itself it is unstable. If e to start a wave of that sort, it would flatten out, because the speed dependence on height that resulted ng in the case we di 51-3 Waves in solids The next kind of waves to be discussed are the more complicated waves in direct analog to a sound wave in a solid. If a sudden push is applied to a solid, it is compressed. It resists the compression, and a wave analogous to sound is started However there is another kind of wave that is possible in a solid, and which is not possible in a fluid. If a solid is distorted by pushing it sideways(called shearing), then it tries to pull itself back. That is by definition what distinguishes a solid from a liquid: if we distort a liquid (internally), hold it a minute so that it calms down and then let go, it will stay that way, but if we take a solid and push it, like shearing a piece of"Jello, and let it go, it flies back and starts a shear wave, travelling in the same way the compressions travel. In all cases, the shear wave speed is less than the speed of longitudinal waves. The shear waves are somewhat more anal- ogous, so far as their polarizations are concerned, to light waves. Sound has no polarization, it is just a pressure wave. Light has a characteristic orientation per- In a solid, the waves are of both kinds. First, there is a compl n wave analogous to sound, that runs at one speed. If the solid is not crystalline, then a shear wave polarized in any direction will propagate at a characteristic speed (Of course all solids are crystalline, but if we use a block made up of microcrystals of all orientations, the crystal anisotropies average out. Another interesting question concerning sound waves is the following: What happens if the wavelength in a solid gets shorter, and shorter, and shorter? How short can it get? It is interesting that it cannot get any shorter than the space
between the atoms, because if there is supposed to be a wave in which one point goes up and the next down, etc, the shortest possible wavelength is clearly the atom spacing. In terms of the modes of oscillation, we say that there are longi tudinal modes, and transverse modes, long wave modes, short wave modes. As we consider wavelengths comparable to the spacing between the atoms, then the speeds are no longer constant; there is a dispersion effect where the velocity is not independent of the wave number. But, ultimately, the highest mode of transverse vaves would be that in which every atom is doing the opposite of neighboring Now from the point of view of atoms, the situation is like the two pendulums that we were talking about, for which there are two modes, one in which they both go together, and the other in which they go apart. It is possible to analyze the olid waves another way, in terms of a system of coupled harmonic oscillators, like an enormous number of pendulums, with the highest mode such that they oscillate oppositely, and lower modes with different relationships of the timing The shortest wavelengths are so short that they are not usually available technically. However they are of great interest because, in the theory of thermo- dynamics of a solid, the heat properties of a solid, for example specific heats, can be analyzed in terms of the properties of the short sound waves. Going to extreme of sound waves of ever shorter wavelength, one necessarily comes to the individual motions of the atoms; the two things are the same ultimately a very interesting example of sound waves in a solid, both longitudinal and transverse, are the waves that are in the solid earth, who makes the noises we do not know, but inside the earth, from time to time, there are earthquakes-some ck slides past some other rock. That is like a little noise So waves like sound waves start out from such a source very much longer in wavelength than one usu- ally considers in sound waves, but still they are sound waves, and they travel around in the earth. The earth is not homogeneous, however, and the properties of pressure, density, compressibility, and so on, change with depth, and therefore the speed varies with depth. Then the waves do not travel in straight lines---there is a kind of index of refraction and they go in curves. The longitudinal waves and the mm数 way the thing jiggles after there has been an earthquake somewhere else, then we do not just get an irregular jiggling. We might get a jiggling, and a quieting down, and then another jiggling-what happens depends upon the location. If it were close enough, we would first receive longitudinal waves from the disturbance, and then, a few moments later, transverse waves, because they travel more slowly. By measuring the time difference between the two, we can tell how far away the earth- quake is, if we know enough about the speeds and composition of the interior egions involved ANSVLA An example of the behavior pattern of waves in the earth is shown in Fig. 51-6 The two kinds of waves are represented by different symbols. If there were an earth Fig. 51-6. Schematic of the earth quake at the place marked"source, "the transverse waves and longitudinal waves longi would arrive at difTerent times at the station by the most direct routes, and there verse sound waves would also be reflections at discontinuities, resulting in other paths and times. It turns out that there is a core in the earth which does not carry transverse waves. If the station is opposite the source, transverse waves still arrive, but the timing not right. What happens is that the transverse wave comes to the core, and when er the transverse waves come to a surface which is oblique between two materials, two new waves are generated, one transverse and one longitudinal. But inside the core of the earth, a transverse wave is not propagated (or at least, there is no evi- dence for it, only for a longitudinal wave); it comes out again in both forms and comes to the station It is from the behavior of these earthquake waves that it has been de that transverse waves cannot be propagated within the inner circle. This means that the center of the earth is liquid in the sense that it cannot propagate transverse waves.The only way we know what is inside the earth is by studying earthquakes So, by using a large number of observations of many earthquakes at different