fluids q

[greenseeking]

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which of the following is true concerning an aluminum buoy that floats in a lake all year round?

A. buoy floats higher in the winter b/c density of water changes more than density of aluminimum
B. buoy flaots higher in winter b/c the density of water changes less than density of al
C. buoy floats lower in the winter b/c density of water changes more than density of al
D. buoy floats lower in winter b/c the density of water changes less than density of al.

Given that the coefficient of volume expansion (beta) for water is bigger (2 * 10^-3) than the coefficient of Aluminimum (7 * 10^-5) . delta V= V (beta) DeltaT
beta for ice at 0C is 1.53 * 10 ^-4

ANSWER is A.

I eliminated B/D easily b/c of the beta coefficient.
I chose C because for water, density of ice (in the winter) is less than density of liquid water (in summer). So I figured that since mg (of al) = rho(H2O) V(H20 diplaced) g ; mg stays constant; Since rho of water in winter (ice) is smaller, then by D=M/V Volume displaced should be bigger. Thus, the buoy floats lower in the winter.

Am I wrong in assuming that the water in the winter time would be ice-water?

 

[mehc012]

which of the following is true concerning an aluminum buoy that floats in a lake all year round?

A. buoy floats higher in the winter b/c density of water changes more than density of aluminimum
B. buoy flaots higher in winter b/c the density of water changes less than density of al
C. buoy floats lower in the winter b/c density of water changes more than density of al
D. buoy floats lower in winter b/c the density of water changes less than density of al.

Given that the coefficient of volume expansion (beta) for water is bigger (2 * 10^-3) than the coefficient of Aluminimum (7 * 10^-5) . delta V= V (beta) DeltaT
beta for ice at 0C is 1.53 * 10 ^-4

ANSWER is A.

I eliminated B/D easily b/c of the beta coefficient.
I chose C because for water, density of ice (in the winter) is less than density of liquid water (in summer). So I figured that since mg (of al) = rho(H2O) V(H20 diplaced) g ; mg stays constant; Since rho of water in winter (ice) is smaller, then by D=M/V Volume displaced should be bigger. Thus, the buoy floats lower in the winter.

Am I wrong in assuming that the water in the winter time would be ice-water?

Yes. The question is asking about how high/low the buoy would float. If the water freezes, the buoy would no longer be floating. You don't talk about a book 'floating' on a table!

 

[RNtoMD2016]

Yes. The question is asking about how high/low the buoy would float. If the water freezes, the buoy would no longer be floating. You don't talk about a book 'floating' on a table!

This is not the right thinking at all! This question has absolutely nothing to do with an object not being able to technically "float" on ice.

0P, you were only incorrect in thinking that because ice is less dense than liquid water, cold water is less dense than warm. When water gets warm, it expands. The same mass of water fills more volume in the summer than in the winter, and is thus less dense according to d=mv. Therefore water in the winter is MORE dense and the object floats higher. It's a poorly written question because normal people equate winter with frozen water, not cold water.

 

[mehc012]

This is not the right thinking at all! This question has absolutely nothing to do with an object not being able to technically "float" on ice.

0P, you were only incorrect in thinking that because ice is less dense than liquid water, cold water is less dense than warm. When water gets warm, it expands. The same mass of water fills more volume in the summer than in the winter, and is thus less dense according to d=mv. Therefore water in the winter is MORE dense and the object floats higher. It's a poorly written question because normal people equate winter with frozen water, not cold water.

My point was only that the water does not freeze, and therefore they should be looking at the density of cold water, not the density of ice.
But thanks for randomly necrobumping a thread to help an OP who has surely finished the MCAT by now, just to tell me that my thinking was wrong when we're in 100% agreement.

 

[Cawolf]

You can always tell a good poster by the quality of their first posts! 🙂

 

[RNtoMD2016]

My point was only that the water does not freeze, and therefore they should be looking at the density of cold water, not the density of ice.
But thanks for randomly necrobumping a thread to help an OP who has surely finished the MCAT by now, just to tell me that my thinking was wrong when we're in 100% agreement.

Hmm. I didn't reply to necessary help the poster, but to help clarify for those who search for clarifications to similar questions (as this is the reason I found the post in the first place). I interpreted your response a completely different way since you didn't say anything about cold water or density. Your response in both instances is less than helpful!

 

[RNtoMD2016]

You can always tell a good poster by the quality of their first posts! 🙂

You can always tell a good English speaker by their use of the word good!

 

[Cawolf]

You can always tell a good English speaker by their use of the word good!

What word do you think I should have used? Is there a better adjective to describe you bumping a year old thread to post a poor answer?

 

[Cawolf]

cold water is less dense than warm

Wrong. Look up a density of water versus temperature chart.

according to d=mv

Is this a proprietary formula for density I don't know about?

 

[RNtoMD2016]

Then I'm confused! Hence I would benefit from some actual help on this problem, not just trolling. And yes d=m/v, but you and I both know that was a typo.

 

[Cawolf]

I am just going to un-watch this thread and assume that your posts are the result of a language/cultural barrier.

Best of luck.