a friend and i were having a heated discussion about this while going to college one day
we travel everyday by train so here is what it was all about
scenario 1)
ur travelling at 10 m/s in a train. u throw a stone out at an initial vel of 0m/s. it will travel at 10 m/s and reach the ground.
scenario 2)
ur travelling at 10 m/s in a train. u throw a stone out at an initial vel of 10m/s. it will travel at 20 m/s and reach the ground.
simple relative velocity.
scenario 3)
now ur travelling in the train at around 200m/s. u stand at the door and shout. we know that sound travels at 330m/s (approx). but because of the train’s velocity, will the sound travel at 330+200=530 m/s?
that would mean that sound is breaking the sound barrier
the sound travels at the speed of sound. This is the fundament of the doppler effect. The frequency will be higher in the direction of travel and lower from behind. Doppler… look into it.
If you’re standing inside the train, and the train is closed and isolated, you will hear no difference compared to what you would hear if the train were standing.
Similar to, say, the Concorde: Even if it’s travelling with a speed of Mach 2, it’s of course still perfectly possible to talk inside, so one can justly say the speed travelled at Mach 1 inside the plane and at Mach 3 relatively to the non-moving ground.
As far as I know, the speed of sound is not as constant as the speed of light is, which is always 300,000 kilometres per second, no matter whether you are standing, moving of whatever.
The speed of sound is a property of the medium through which it is travelling, as it is a vibration. If you shout out the side of a train, the sound waves will travel at a speed determined by how quickly the air molecules bump into each other. The actual wavelengths will differ from normal, though, as each wave will propagated from a different location. Thus the doppler effect, which accounts for police sirens sounding different while approaching than while leaving.
I read about an experiment which created the impression that a physician had been able to speed an electromagnetic wave to a speed 4.2 times the “classic” speed of light, but another scientist argued this observation resulted from a misinterpretation: They regarded the poinst of the wave with maximum amplitude, which seemed to travel faster than light, but if one regards the points of zero amplitude, the speed apparently still is at 300,000 km/sec.
As far as I know, the question whether Einstein’s dogma that the speed of light is the absolutely constant thing in universe is right or not, has not yet been answered.
You are thinking of the speed of light in a vaccum. Light actually travels slightly slower in air.
Schnitte - obviously you can talk in concorde, its got nothing to do with the speed of the plane, as the air inside the cockpit is still. You wouldnt be able to talk if the cockpit were open to the air. If you stood at the back of concorde when it was doing mach 1 and shouted, obviously the sound would travel up the plane as the air was still, and technically it is travelling faster than the plane, but the air molecules are not actually travelling faster. The limit on the speed of sound is just to do with how fast the molecules of air can move and bump into one another. Which is why sound travels much faster in liquids and solids.
My cite quotes a physicist that thinks the speed of light has been gradually slowing over time since the big bang. That’s a whole 'nuther issue. There are several aspects to the suspected variability of the speed of light, but I’m not adept enough at quantum physics or superstring theory to evaluate them.
Sure. I know that the thing that really moves the sound is a wave of changing air pressure.
This is why I wrote “if the plane is closed and isolated”. But if you are an observator standing at the non-moving ground, for you there’s the impression of sound travelling at Mach 3, since for you the signal of the sound wave has made three times the distance it would have made if the plane stood still. For you, the signal would, after one hundreth of a second, be 9.9 metres away from the point where it was emitted. It travelled 3.3 metres inside the Concorde, and since the Concorde changed its position by 6.6 metres (at Mach 2) during the same timespan, the signal has travelled 9.9 metres, even if only 3.3 metres of this really were done by the sound itself. The sound, being a movement of the air inside the plane, is being carried by the plane. The speed of the sound itself of course has not changed. This is why I think we have to draw a distinction between “speed of the signal for an observator inside the plane” (Mach 1) and “speed of the signal for a non-moving observator outside the plane” (Mach 3)
With scenario 3, wow that is a FAST train. Well, provided you survive the force of the wind in your face any sound you make will be quickly blown behind you and you will create a neat little sonic boom in a cone shape around the train. If you hang around Air Force bases long enough you will see jets that break the sound barrier. And if you stand in the right place, so that they are traveling towards you, you wont hear a thing. If that jet is traveling 3 times the speed of sound and it passes 1,000 feet directly over you, you still wouldn’t hear a thing. It would take about a second after that plane flew over you till you hear that sonic boom. But if it was say a 737, you would hear it well before it got close to you.
As for those light comments, the speed of light is always c, always. Even in air, it only appears to go slower due to the fact that it begins to be absorbed by various molecules, held for a bit and then released. But as those photons travel between those atoms and molecules it goes directly at c. That makes sense right? What exists between all the atoms and molecules in our athmosphere? empty space, vacuum. Anybody with a PHd cn back me up or tell me I’m wrong? I made this assumption taking upper level physics courses.
So you’re travelling in a car, and the window is slightly open. A bee is zipping around outside and manages to fly right through theopen window. Would the speed of its flight suddenly be so fast it’d crash into the inside of the windscreen?
I doubt this is really related much, though it kind of is.
has a couple of nice animations illustrating the Doppler effect and the supersonic shock wave - just click on the relevant links.
I found it interesting that the shockwave travels faster than the speed of sound - at the same speed as the supersonic object. I wonder how long it takes that shockwave to decelerate into soundwaves when the supersonic object drops below the sound barrier?
brother rat: you’re describing the supersonic case:-in zub’s scenario 3 the train is still subsonic (200 m/s is about mach 0.6).
GuanoLad If anything, the bee is in danger of crashing into your rear window. Say you’re doing 60 mph, then if Mr bee manages to slip through that turbulent air blasting around your car, he’s going to find himself in a “block” of air moving at 60 mph relative to him with the rear window coming up just as fast. The question is, will that moving air accelerate him to match speeds with the rest of your car before the window gets him?
You could find out by firing bees at precise velocities out of a compressed air gun into still air, and tracking their deceleration with high speed photography. I’d volunteer, but I’ve found myself being uncharacteristically nice to insects recently - I can’t even smack wasps out my bedroom window with a tennis racquet any more.
The sonic boom shockwave is a sound wave, and as such, always travels at the speed of sound. The point of origin of the shockwave, however, is the plane, and hence is moving faster than the speed of sound. There’s alaways a cone with its apex at the plane, but it’s not always the same cone, so to speak.
As to light, it’s not even meaningful to talk about the speed of light changing. If, somehow, the speed of light did change, then that would cause corresponding changes in every conceivable device for measuring length and/or time, in just such a way that the speed of light would be measured as the exact same value. The same is true, for instance, of Planck’s constant, or Boltzman’s constant. Fundamental constants of nature are funny that way.
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I was led to understand at one point that one can in fact create an ‘optical boom’ analogous to a sonic boom. The way you do this is to take some medium (e.g., a tank of water), and introduce something that is travelling faster than the speed of light in water (but obviously, not as fast as the speed of light in a vacuum). Would any of the Real Physicists care to expound on this one?
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Chronos: “The sonic boom shockwave is a sound wave, and as such, always travels at the speed of sound. The point of origin of the shockwave, however, is the plane, and hence is moving faster than the speed of sound. There’s alaways a cone with its apex at the plane, but it’s not always the same cone, so to speak.”
I’ve always found shockwaves confusing, and what little I know about them was gathered from somewhat sparse web sources, so bear with me…
If you’re saying that the cone isn’t a “real” phenomenon but maps the wavefronts of the soundwaves emitted by the supersonic body, I guess I can see that from the animation I linked to earlier. And sound waves result in no net flow of gas, which makes sense a long way from the supersonic body.
However, at the supersonic body itself, you have rigid surfaces moving through the medium faster than the medium can transmit pressure away. So you must have a “piling up” of compressed air ahead of the body. This seems to be a similar situation to the Becker model for a piston moving in a gas-filled tube, as described in the link below.
The same site describes the shock wave as analagous to the bow wave of a boat travelling faster than the speed of surface waves. It may be true that such a bow wave can be regarded as a combination of surface wave wavefronts, but it appears to be a very real abrupt wave when it kicks me off my sailboard, which has happened a few times. And it moves at the same speed as the boat.
Could you elaborate on the nature of a shockwave, and explain how a sonic-boom shockwave relates to the Becker model, if at all? Or I could post a new OP in GQ on the subject if that’s more appropriate.
I think you’ve got it, matt. We’re just using slightly different terminology: You were using “shockwave” to refer to the cone, while I was using it to refer to the individual wavefronts that made it up.