MJL21193 said:
You're only looking at one side of the motion. Its action/re-action...in action. If I have A|||B|||C, then introduce an action such that /A||B|||C occurs, I have locally increased the density of A and B (not affecting C). However A and B would rather be ||| apart, so this occurs next: /A|||B||C (because B has some inertia). But B and C want to be ||| apart, so it becomes: /A|||B|||C and everybody has moved forward by '/'. It is the speed that the || becomes ||| that dictates how quickly speed will move through the substance.
MJL21193 said:
But when was your last cold beer, milk, ice cube...Its about where you observe and what you relate your observation to.
MJL21193 said:
Waitaminute, which way did speed go? Up or down, your statement is ambiguous.
To comment anyway: You should see the speed go down, not strictly because density went down but because compressibility increases with temperature. Increased compressibility translates to a reduced desire to resist a change and consequently slow the propagation of the pressure wave.
Daveze said:
But this only proves my point - simple hydraulics, transfer of energy from the bigger piston (the bottles end) and the smaller piston (the cap). Mechanical advantage, that's what hydraulics are about. Using the relative incompressibility of a fluid.
When you fill something completely with water, for sound transmission purposes, it behaves like a solid. The container that holds the water and the water inside will conduct sound - no compression. Just like a solid hunk of steel.
MJL21193 said:
But it doesn't. Even solids don't act as solids. It will take a finite period of time for the bottle to break at the bottom after you break it at the top. Even a photon will take a finite period of time to travel the distance from the top of the bottle to the bottom. To say that the liquid is completely incompressible, and will hence translate the force instantaneously, is in defiance of Einstein's Theory of Special Relativity.
Actually water is a very naughty substance, it expands both below 4ºC and above 4ºC having a maximum density point at that temperature. It's not a linear medium, it's likely to distort sound (like air but worse).
If you want to read on water anormalities (I strongly recommend you to read this):
http://www.lsbu.ac.uk/water/explan2.html
Anyway, since you have agreed that water volume changes with themperature (across the liquid phase), you have already agreed that water may be compressed and expanded (by changing temperature).
Indeed, when sound propagates through water, the resulting compressed and the expanded zones are likely to have different temperatures (as it happens with air).
If you want to read on water anormalities (I strongly recommend you to read this):
http://www.lsbu.ac.uk/water/explan2.html
Anyway, since you have agreed that water volume changes with themperature (across the liquid phase), you have already agreed that water may be compressed and expanded (by changing temperature).
Indeed, when sound propagates through water, the resulting compressed and the expanded zones are likely to have different temperatures (as it happens with air).
Daveze said:
Invoking Einstein, eh?
Eva said:
What is waters natural state? Liquid, is it not? Water that has been heated to the boiling point does not compress back to water, it condenses. To do this it's temperature needs to drop. Not outside force is needed, therefore no compression.
More grade school oversimplification, I'm sure.
MJL21193 said:
That depends entirely on it's environment.
MJL21193 said:
Compressing a substance increases its temperature (adding energy), and thus cooling a substance can lead to contraction (removing energy). So you can compress a substance by cooling.
MJL21193 said:
Your favorite, cozy cozy world. It helps you feel safe.
MJL21193 said:
Yes.
MJL21193 said:
Yes, more grade school oversimplification. A method of teaching things is to simplify things such that they are palatable to a mind at its state of maturity, then gradually un-simplify things as the mind's understanding matures.
Example: Gravity
Primary school: Gravity holds you to the ground.
Early high school physics: Gravity is the acceleration that the Earth imposes on other objects = ~10 or 9.81m/s^2
Later physics: Gravity is an interaction between two bodies with mass. The magnitude given by G(m1+m2)/r^2.
Cutting edge physics: Gravity has something to with a theoretical particle that I don't come close to understand.
Water does not have a 'natural state'. What it (and all other substances) does is have a state/s according to a set of conditions: Temp and Pressure.
AndrewT said:
And if this very unique characteristic of water did not occur, life would not exist on the earth. Ice would sink when frozen and eventually all of the water would turn into ice thus killing off the evolving species. Its the ice layer that insults the water below. This has always amazed me as truely "an act of God".
water is compressible.
and you can trade pressure for temperature just like with a gas.
that's why when you go camping in the mountains, your pasta takes longer to cook--water boils at a lower temperature due to the lower atmospheric pressure so you're boiling with less heat.
if you compress water enough, it'll solidify. or if you can't do that, just lower the temperature.
the compression is VERY SMALL. same in other solids. just because you can't see it doesn't mean it's not there.
sound also travels through steel for the same reason. minute rarefactions and compressions through the material.
the compression of a solid doesn't have to be at a constant temperature. there's no constraint that says it must. when you play music in a pool, the temperature changes as the compression waves travel through it. those changes are also very small and quickly diffuse into the large thermal sink.
Here's some more detail on my comment in the other thread. A quote from a nasa page if you like your science hard:
http://earthobservatory.nasa.gov/Library/GlobalWarmingUpdate/global_warming_update6.html
Notice the clear differentiation between adding water from glaciers (RIP) and expansion of existing water due to increasing global temperatures.
and you can trade pressure for temperature just like with a gas.
that's why when you go camping in the mountains, your pasta takes longer to cook--water boils at a lower temperature due to the lower atmospheric pressure so you're boiling with less heat.
if you compress water enough, it'll solidify. or if you can't do that, just lower the temperature.
the compression is VERY SMALL. same in other solids. just because you can't see it doesn't mean it's not there.
sound also travels through steel for the same reason. minute rarefactions and compressions through the material.
the compression of a solid doesn't have to be at a constant temperature. there's no constraint that says it must. when you play music in a pool, the temperature changes as the compression waves travel through it. those changes are also very small and quickly diffuse into the large thermal sink.
Here's some more detail on my comment in the other thread. A quote from a nasa page if you like your science hard:
http://earthobservatory.nasa.gov/Library/GlobalWarmingUpdate/global_warming_update6.html
Notice the clear differentiation between adding water from glaciers (RIP) and expansion of existing water due to increasing global temperatures.
John, you proposed another inapt (DC) analogy instead of answering my point. There are two ways of defining compression and, as I explained, the peaks of any acoustic wave satisfy both of them. You need to show how the First Law doesn't apply, remembering that for every compression, there's a rarefaction.
Interesting discussion.
I'm with SY on this one. Some people can't tell the ACs from the DCs.
I can certainly see MJL's point though. Water doesn't compress, for general practicle purposes. It's easy to get more air into a certain volume, like a car tire, a scuba tank, a V8 engine. Stuffing more water into the same those spaces would be a lot harder.....
But on a local scale, there can be compression - such as in a sound wave. There has to be rarification too, so the sum is no overall compression of the liquid water.
I'm with SY on this one. Some people can't tell the ACs from the DCs.
I can certainly see MJL's point though. Water doesn't compress, for general practicle purposes. It's easy to get more air into a certain volume, like a car tire, a scuba tank, a V8 engine. Stuffing more water into the same those spaces would be a lot harder.....
But on a local scale, there can be compression - such as in a sound wave. There has to be rarification too, so the sum is no overall compression of the liquid water.
In hydrodynamics and fluid dynamics (DC) water is assumed to be incompressible because its changes through compression are miniscule compared to the other effects. But in acoustics (AC) one cannot make this simplification because it would mean that sound could not propagate. The two fields of study are almost always completely uncoupled from each other and results from one are usually not applicable to the other. It is precisely the incompressible assumption that accounts for this.
MartinQ said:
Nearly 4 hours in the dentists chair and I have to come back to this?
She's a pretty 27 year old though, and it was well worth the pain, irritation and fiscal hardship. 
Bad news boys, she agreed with me.
The environment? Here on planet Earth? Water is in a liquid state that covers 71% of the Earths surface, that would qualify as natural.
So, you can compress a substance by cooling. I said that already.
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