Physics

[neil]

Don't forget to convert to the Kelvin scale.

Calvin?
http://calvinandhobbes.michaelgoonan.net/Images/calvin2.gif

 

[dumpsterDiver]

How did you do with those type questions on the DM physics exam or on the pre-test in your IDC? They don't account for depth anyplace.

Water isn't very compressible so the density is fairly constant with depth and the questions are usually worded so as to ask how much water needs to be displaced in order to get neutral or to a specific positive buoyancy so how far off was their answer?

I don't remember the details, but if they ask you how many cu-ft of air is required to provide a certain amount of lift, then the depth of the water matters a lot due to the compressibility of air, certainly fluids can be assumed to be incompressible in this type of problem. I think that the DM questions/answers were correct.

 

[MikeFerrara]

I don't remember the details, but if they ask you how many cu-ft of air is required to provide a certain amount of lift, then the depth of the water matters a lot due to the compressibility of air, certainly fluids can be assumed to be incompressible in this type of problem. I think that the DM questions/answers were correct.

The questions are usually worded so as to ask the volume of water that must be displaced. Like the question in this thread.

An Object weighs 237kg and displaces 123 litres of water. How much additional seawater do you need to displace to give the object 40 kg positive buoyance

They aren't asking how much gas is needed to displace the water...just what volume of water must be displaced. We don't need to know the depth to answer this question.

If the question asked how much air at some reference pressure would be required to displace that volume of water, then you absolutely do need to know the depth.

 

[Thalassamania]

The questions are usually worded so as to ask the volume of water that must be displaced. Like the question in this thread.



They aren't asking how much gas is needed to displace the water...just what volume of water must be displaced. We don't need to know the depth to answer this question.

If the question asked how much air at some reference pressure would be required to displace that volume of water, then you absolutely do need to know the depth.

But you can't answer that without knowing the density of the water, a function of the salinity, temperature and yes … even the depth (to a very small degree).

 

[MikeFerrara]

But you can't answer that without knowing the density of the water, a function of the salinity, temperature and yes … even the depth (to a very small degree).

Yes temp and salinity have an effect on density and we need to know the density.
However, The assumed density is given in the text or even on the test and the purpose of these questions is for the student to demonstrate their understanding of Archimedes principle. No dimensional analysis or vector diagrams needed

 

[Thalassamania]

Yes temp and salinity have an effect on density and we need to know the density.
However, The assumed density is given in the text or even on the test and the purpose of these questions is for the student to demonstrate their understanding of Archimedes principle. No dimensional analysis or vector diagrams needed

Shucks ... potential density was one of my favorite topics.

 

[h90]

But you can't answer that without knowing the density of the water, a function of the salinity, temperature and yes … even the depth (to a very small degree).

At http://www.es.flinders.edu.au/~mattom/Utilities/density.html is a calculator. change in pressure has not really an impact, even at 12.000 meter under water it is not much

 

[Thalassamania]

At http://www.es.flinders.edu.au/~mattom/Utilities/density.html is a calculator. change in pressure has not really an impact, even at 12.000 meter under water it is not much

It is of interest to an Oceanographer, and its impact while appearing small in magnitude is part of what drives the planet's weather. Ignoring it would result in gross misunderstanding of the oceans' circulation. You don’t think we keep it in there just for the fun of it, do you?

 

[h90]

It is of interest to an Oceanographer, and its impact while appearing small in magnitude is part of what drives the planet's weather. Ignoring it would result in gross misunderstanding of the oceans' circulation. You don’t think we keep it in there just for the fun of it, do you?

Of course it is very important for oceans and lakes as well, but maybe a bit less of interest for someone who don't know if he must add or deduct the 40 kg in the original question.
I must point out again that is nothing like a 40 kg positive buoyance, what is meant here is a buoyancy of 40x9.80665 Newton. 40 kg are also on the moon 40 kg, but here is mean the force 40 kg are causing (by the way not everywhere the same on the planet).

 

[Thalassamania]

Ah yes, mass doesn't change with gravity or buoyancy but weight sure does. Shall we slug it out? Or perhaps just get stoned?