Why can't a liquid move faster than the speed of sound in that medium?

Submitted ⁨⁨8⁩ ⁨months⁩ ago⁩ by ⁨makeshiftreaper@lemmy.world⁩ to ⁨nostupidquestions@lemmy.world⁩

I was watching an XKCD “What-If” video recently and Randal off-handedly mentions the title fact as a given. Upon a further Google search I see explanations about why sound moves faster in liquids than gasses but nothing for my specific question. Is there an intuitive explanation for that fact or is it just one of those weird observable facts with no clear explanation

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UnderpantsWeevil@lemmy.world ⁨8⁩ ⁨months⁩ ago
Might help to look up the definition of Terminal Velocity.

Terminal velocity is the maximum speed an object can reach while falling through a fluid (like air or water). It occurs when the force of gravity pulling the object down is balanced by the fluid’s resistance, or drag, pushing it up.

As the speed of an object increases, so does the drag force acting on it, which also depends on the substance it is passing through (for example air or water). At some speed, the drag or force of resistance will be equal to the gravitational pull on the object. At this point the object stops accelerating and continues falling at a constant speed called the terminal velocity (also called settling velocity).

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Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
The biggest irony is not being able to rely on search engine results because it’s just explanations about why sound moves faster in a liquid and not any answers to your question, only to ask on Lemmy and have people explain why sound moves faster in a liquid and not give any answers to your question.

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Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
Ahhh I in terms of fluid flow? I mean you can move a glass of water as fast as you want (don’t @ me physicists) if the glass is moving at that speed.

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sem@lemmy.blahaj.zone ⁨8⁩ ⁨months⁩ ago
Does this mean that a drop of water can’t move through air faster than the speed of sound in air?

Or that a drop of water can’t move faster than the speed of sound in water anywhere?

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- Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
  He’s asking why water can’t flow through a pipe at the speed of sound in water.
  
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- meekah@lemmy.world ⁨8⁩ ⁨months⁩ ago
  I’m pretty sure they are talking about the 2nd one
  
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ptolemai@lemmy.world ⁨8⁩ ⁨months⁩ ago
How about firing a compressed air canon to attempt a sonic boom

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macaw_dean_settle@lemmy.world ⁨8⁩ ⁨months⁩ ago
Use a better search like Bing or duckduckgo next time. googol sucks and was never any good. Quit using ignorant garbage.

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- Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
  Like bing? Can’t tell if I’m being baited or not, I’m gonna assume you are and that this is actually a funny comment.
  
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marcos@lemmy.world ⁨8⁩ ⁨months⁩ ago
Just to add because nobody mentioned that yet… But you can always push stuff faster than the speed of sound, it will just stop being a liquid, and probably explode, but there’s no law saying the material can’t go faster.

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- UnderpantsWeevil@lemmy.world ⁨8⁩ ⁨months⁩ ago
  
  But you can always push stuff faster than the speed of sound
  
  Sure, but said stuff can push back at you in turn. At some point, you reach an equilibrium between the force applied to the material and the reflected force due to resistance.
  
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rikudou@lemmings.world ⁨8⁩ ⁨months⁩ ago
You already got some answers, but I thought of something you might find interesting: if you had a multiple kilometers long pole in a vacuum and pushed on it, the push itself would propagate at the speed of sound!

Meaning the other end wouldn’t really move immediately, but it would instead take multiple seconds or even minutes if the pole is large enough. If it’s made of oak and is 9 km long, it would take around 3 seconds (the speed of sound in oak is around 3 km/s IIRC).

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- xavier666@lemmy.umucat.day ⁨8⁩ ⁨months⁩ ago
  I think this was experimentally shown by a Youtuber. The speed of sound is a slight oversimplification since there are multiple types (engineeringtoolbox.com/sound-speed-solids-d_713.h…)
  
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- pineapplelover@lemmy.dbzer0.com ⁨8⁩ ⁨months⁩ ago
  I think Randall mentioned this at one point but I never really understood it. Is it something like on a molecular level they’re still taking some time to push in to each other? It’s so damn trippy. At what point is my long pole going to have a delay when I push it? It sounds unreal
  
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  - rikudou@lemmings.world ⁨8⁩ ⁨months⁩ ago
    Basically, when you push something, you push molecules, those in turn push the other molecules etc., that’s what it is.
    
    The delay is there every time, it’s just usually really fast, the speed of sound in solid mediums is much bigger than the speed of sound in air.
    
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- ryannathans@aussie.zone ⁨8⁩ ⁨months⁩ ago
  Do incompressible materials therefore have extremely high speed of sound?
  
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  - eRac@lemmings.world ⁨8⁩ ⁨months⁩ ago
    Yes. Nothing is truly incompressible. The speed of sound can be viewed as a measure of how much a material can squish on the atomic level before the next atoms move.
    
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octoshrimpy@sh.itjust.works ⁨8⁩ ⁨months⁩ ago
Imagine the liquid as a road, and the sound travelling through it as a runner on said road. Now, sound is faster than the road, because road isn’t moving anywhere. But if we replace the road with one of those airport speedwalking walking pads (kinda like escalators but flat on ground), now the ground is moving, but as luck would have it, the runner is now moving even faster. The more you speed up the pad, the faster the runner moves, even if the runner themselves has not increased their speed.

Weird stuff, but it does make sense. :)

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- Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
  This analogy has nothing to do with why fluids can’t flow faster than their internal speed of sound
  
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Little_mouse@lemmy.ca ⁨8⁩ ⁨months⁩ ago
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.”

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- Fleur_@aussie.zone ⁨8⁩ ⁨months⁩ ago
  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?
  
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  - Little_mouse@lemmy.ca ⁨8⁩ ⁨months⁩ ago
    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.
    
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- dohpaz42@lemmy.world ⁨8⁩ ⁨months⁩ ago
  So the medium is like a car made of liquid and the speed of sound is a passenger?
  
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  - DontAskAboutUpdog@lemmy.world ⁨8⁩ ⁨months⁩ ago
    No, medium is the speed limit and the sound is a car that drives legaly.
    
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- makeshiftreaper@lemmy.world ⁨8⁩ ⁨months⁩ ago
  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?
  
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  - knightly@pawb.social ⁨8⁩ ⁨months⁩ ago
    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 water 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.
    
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  - halcyoncmdr@lemmy.world ⁨8⁩ ⁨months⁩ ago
    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.
    
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Nemo@slrpnk.net ⁨8⁩ ⁨months⁩ ago
So, sound is vibration, right? And it’s going to vibrate as fast as it can, at the fastest rate that liquid can move. So it’s not that the rate of movement comes from the speed of sound; if anything, it’s the reverse.

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- Zwuzelmaus@feddit.org ⁨8⁩ ⁨months⁩ ago
  
  it’s going to vibrate as fast as it can, at the fastest rate that liquid can move.
  
  Not quite logical unless there’s a reason why it should vibrate as fast as it can
  
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