ipodtouch

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Sep 18, 2008
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According to the formula

v=(Young's Modulus/Density)^-0.5

Greater density means lower speed.


The only difference between salt water and freshwater is density. Why does sound travel faster in a denser liquid??
 
Nov 22, 2011
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According to the formula

v=(Young's Modulus/Density)^-0.5

Greater density means lower speed.


The only difference between salt water and freshwater is density. Why does sound travel faster in a denser liquid??
Stick with your original instinct on this one because unless the exact values are given, it is best to stick with density and forget bulk modulus all together... in terms of the MCAT only of course.

Sound travels slower in denser medium of the same phase (solid, liquid or gas) almost always. Bulk modulus changes with temperature and pressure as does density in this case so it would actually depend on the condition of the experiment as to whether salt water or fresh water wins out.

For the purposes of the MCAT, I would use common knowledge or common sense about density properties only as your foundation for comparing unless given a passage about bulk modulus and changes in bulk modulus vs temp.

The bulk modulus of salt water and fresh water is not the same
 
Feb 28, 2012
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How and when can you tell which has the greater effect on speed of sound? athe bulk modulus or density? I know that within the same phase density has the greater effect i.e. speed of sound is greater in He than O2. But for this example, isn't freshwater and saltwater both in liquid phase, so shouldn't we only consider density? why is bulk modulus more important here?
 

chiddler

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Apr 6, 2010
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for this question, i'm almost certain you need more information to answer it with confidence. it is not really common sense, with our humble physics abilities, to predict that adding salt to water would change bulk modulus.

speed is proportional to the square root of bulk modulus and inverse square root of density. if you double both, speed of sound remains the same.
 
Jul 9, 2011
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How and when can you tell which has the greater effect on speed of sound? athe bulk modulus or density? I know that within the same phase density has the greater effect i.e. speed of sound is greater in He than O2. But for this example, isn't freshwater and saltwater both in liquid phase, so shouldn't we only consider density? why is bulk modulus more important here?
it's not that bulk modulus is more important, but that bulk modulus changes MORE than density does.

The ratio of effect should be 1:1. But it's just difficult to know how much bulk modulus changes without given values.


But why do we keep saying "Bulk Modulus?"

The formula has Young's Modulus (which is tensile modulus), not bulk...
 
Apr 25, 2011
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it's not that bulk modulus is more important, but that bulk modulus changes MORE than density does.

The ratio of effect should be 1:1. But it's just difficult to know how much bulk modulus changes without given values.


But why do we keep saying "Bulk Modulus?"

The formula has Young's Modulus (which is tensile modulus), not bulk...
The formulae for speed of sound in a liquid and in solid rod are essentially the same, except that the liquid equation uses bulk modulus and the solid rod equation uses Young's modulus.

http://en.wikipedia.org/wiki/Speed_of_sound#Speed_of_sound_in_liquids
http://en.wikipedia.org/wiki/Speed_of_sound#Long_rods

We should have been using bulk modulus all along.
 
Nov 22, 2011
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It wouldn't make much sense that a tensile property would dictate a compressive motion now would it!? :p
 
Jul 9, 2011
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It wouldn't make much sense that a tensile property would dictate a compressive motion now would it!?
Wait, I'm just echoing BR.

Are we saying that the equation should be Bulk Modulus and NOT young's modulus?
---> BR has it wrong?