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Supersonic wave in a solid 
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#19
Jun2604, 06:45 AM

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PF Gold
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Well guys, I was just referring to some proyectile penetrating supersonically (i.e. v>>c(solid)) into a solid. What is the difference with an aeroplane flying supersonically in the air?. In both mediums there will be elastic or plastic deformations transported via waves. The problem here is: will a solid's particle be deformed if the source of deformation (proyectile) goes faster than elastic wave speed?. Surely it will be deformed, but not in the classical way. My question is if all physical process involved with discontinuities (shock waves) in fluids are extensible for solids. (I'm sure it will be with another equations).
The other question was: what is the role of light velocity in the propagation of deformations?. We always talk about sound velocity, but the real question is: Is it not curious that light velocity does not appear in the Wave Equation? (for elastic waves in a solid). 


#20
Jun2604, 10:08 AM

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PF Gold
P: 39,311

You specifically asked about a wave traveling faster than the speed of sound in the solid. That is impossible by definition. It is, of course, possible for an object to move through a solid faster than the speed of sound (i.e. the natural wave speed) in that solid. 


#21
Jun2604, 05:09 PM

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PF Gold
P: 1,479

I think it was solved yet that a supersonic wave is impossible (or this is what i've understood). You arrived too late at this discussion I mean. You don't see what the light velocity of the solid has to do with elastic propagation. But I have just said I think is the ultimate vehicle of information transport. I know this sounds so bad, but perhaps I am wrong. Are we talking about vibration model inside the solid so that it has nothing to do with photons or some particles that travels near light speed?. I'm just not a physics, so those who know (or aparently knows) you have permission to kick me if you want. 


#22
Jun2804, 06:25 AM

P: 56

Well, at some point you have virtual photons, gluons and all the other stuff interacting between the molecules, the individual atoms, within those atoms... But they aren't really of importance if you are just interested in the propagation of the shock wave.
If you want to calculate the travel time of a car moving from city A to city B, you don't take the speed of light into account, even though it certainly plays a part in your problem, right? It would make everything extremely complicated and then lead to the same results. You could of course start with, say, Newton and Maxwell and then derive the shock wave equations. You'd run across c every now and then, but eventually it would either cancel out or the assorted terms would be so small that you can safely ignore them. 


#23
Jun2904, 06:36 AM

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Hey Moe!
I was thinking the same!. I mean, as you say Wave Equation must be only a particular equation for wave speed very much slow than light velocity. It would be an insteresting (and very hard) problem to derive a equation that had into account relativistic effects. This is what I was looking for somebody saying me. Has anyone something to reply? 


#24
Jun2904, 09:10 PM

P: 55

In solids the velocity of sound is not unique, for example a shear wave will travel at a different velocity (higher) than a pure compression wave. As a matter of fact flexural waves do not have a well defined velocity as they are dispersive ... the frequency of the wave changes with displacement. Furthermore in two and three dimensional waves in solids the propagation area changes with distance so that a characteristic impedance cannot be defined and the wavelelength (but not the frequency) change during propagation. [itex] \[ \therefore \] [/itex] you will need to define the type of wave in solid before you can ask if a faster wave is possible.
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