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
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?
How about firing a compressed air canon to attempt a sonic boom
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 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.
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.
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.
“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.
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.
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.
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).
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
Do incompressible materials therefore have extremely high speed of sound?
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.
Nothing is truly incompressible.
Exactly. One usually speaks of quasi-incompressibility when the resistance against compression (bulk modulus) is much greater than the resistance against shear (shear modulus), which is oft the case for liquids such as water.
However, water has a lower resistance against compression (2 GPa) than e.g. steel (160 GPa), which is considered a compressive material as its shear modulus is ~ 81 GPa.
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.
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
Use a better search like Bing or duckduckgo next time. googol sucks and was never any good. Quit using ignorant garbage.
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. :)
