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this post was submitted on 20 Jul 2025
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No Stupid Questions
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No such thing. Ask away!
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Sound is transferred through a medium literally as a wave. When you get right down to the core of it, the wave requires movement within the medium to transmit.
So it might help to conceptualize it not as "Liquid cannot move faster than the speed of sound in it's medium" but more like "The speed of sound in a liquid medium is defined by the speed at which energy can propagate in that system, which includes motion."
So you're saying I should view the speed of sound in a medium like the speed of light in a vacuum? That it's the "speed-limit" of how a wave propagates and so trying to exceed it is impossible for a physical wave?
Sort of. The speed of light in a vacuum is the speed of causality, nothing can go faster than the maximum speed at which one part of the universe can effect another.
It is possible for fluids to move faster than the speed of sound in the fluid around it, such as the exhaust products of a supersonic jet engine, but in these cases not all of the fluid is operating like a wave. The core of the jet experiences a laminar flow where all of the fluid is moving in the same direction and at roughly the same speed, like a laser instead of a flashlight. At the boundaries of this laminar flow exists a turbulent region where the fluid interacts with the surrounding medium and is slowed to subsonic speeds.
I don't know how this works, but wouldn't the exhaust of the jet be moving at or slower than the speed of sound relative to the other particles in the exhaust. Sure, compared to other particles it's moving faster, but that doesn't really matter if we're only looking at the exhaust. It's not doing anything differently, though the exhaust and outside particles will have to interact at the boundary.
Precisely. It's those boundary areas where the jet and the medium interact where it gets complicated.
Badically. "Liquid/fluid" and "gas" don't necessarily mean the same thing scientifically as they do colloquially, they're actually very close to the same thing.
Fluid dynamics covers the study of liquids, gasses, and plasmas because they're effectively the same.
Note that the speed of sound isn't a constant across various media. Just like the speed of light isn't a constant on different media. The "speed of light" we usually refer to is specifically in a vacuum. Light travelling through a media like water or a prism actually changes speed, however slight.
The same happens to sound. The speed of sound at altitude is different from sea level for instance, because of the atmospheric pressure difference. And sound doesn't propagate at all in a vacuum because it requires the wave to move molecules, which don't exist in a vacuum.
Both, liquids and gases, are fluids. The main difference is that liquid phases have a free surface, e.g. the level of water in a glas, whereas gases don't. Their surface is equal to the surface of their compartment.
Colloquially, but not in actuality. Light still travels at the same speed technically, but it bounces off particles which makes it take a longer path so it takes longer to get from one point to another, but it's speed is still constant.
If I get the explanation on Wikipedia right, it's not the photon taking a longer path, but the photon is absorbed by electons and re-emitted after a short delay. This effect is what decreases the effective speed of light in a transparent medium.
So the medium is like a car made of liquid and the speed of sound is a passenger?
No, medium is the speed limit and the sound is a car that drives legaly.
What's updog?
Nothing dog, whats up with you?
I saw you hadn't been asked in a while, had to give you the opportunity.
Ahhh not really? Energy can propagate within a medium faster than the speed of sound of said medium. Bullets are kinetic energy moving faster than the speed of sound in air.
Also how does defining the speed of sound in a medium (incorrectly) show that a fluid can't flow faster than its internal speed of sound?
True. You can shoot lasers and such through a lot of things, but I am trying to describe a phenomenon in relatively simple language without getting too bogged down in the technical details. The question was about flowing liquids, so I assumed it was understood that that was what I was talking about.
All you did was define the speed of sound and then state:
"The speed of sound in a liquid medium is defined by the speed at which energy can propagate in that system, which includes motion"
Which is just a reiteration of what the speed of sound is and then an incorrect correlation to motion.
You didn't describe what flow is, how it relates to the speed of sound, or and why flow stops at high flow rates. Your comment doesn't answer the question and it's worse than a nothing burger because it contains misinformation.