Need help with physics

[that_guy (Tony)]

heyy all, i need some help with my physics assignement,
I need a formula for finding density of water using temperature and salinity. Also, i need a fomula for drag underwater

cheers
tony

[renormalised (Carl)]

Go here...

http://oceanworld.tamu.edu/resources...apter06_05.htm

or

http://www.es.flinders.edu.au/~mattom/IntroOc/lecture03.html

[GeoffW1 (Geoff)]

Originally Posted by that_guy

heyy all, i need some help with my physics assignement,
I need a formula for finding density of water using temperature and salinity. Also, i need a fomula for drag underwater

cheers
tony

Hi Tony,

The formula for water density is hard to find because it is empirical, that is, derived from observations, referenced to a density of 1000 kg/cu metre at 4 deg C.

First get density 'rho' at a certain temperature T:

rho = 1000(1 - (T+288.9414)/(508929.2*(T+68.12963))*(T-3.9863)^2)

then get saline density 'rhos' at a salinity S:

rhos = rho + AS + BS^(3/2) + CS^2

where:

rho and rhos in kg/cu metre
T in deg C
S in g/kg dissolved (seawater is about 35 g/kg)

A, B, C are coefficients

A = 8.24493E-1 - 4.0899E-3*T + 7.6438E-5*T^2 -8.2467E-7*T^3 + 5.3675E-9*T^4

B = -5.724E-3 + 1.0227E-4*T - 1.6546E-6*T^2

C = 4.8314E-4

Here is the reference you will need:

http://www.earthwardconsulting.com/density.xls

The formula for drag in water (or any fluid including air) is well known, due to Lord Rayleigh:

http://en.wikipedia.org/wiki/Drag_equation

Typical values for drag in water would be:

density rho at 20 deg C : 998 kg/cu metre

'reference area' is approximately cross sectional area, so anything you like

'drag coefficient' Cd depends on shape. For:

sphere 0.47
cylinder 0.82
flat plate at 90 degrees 1.17
dolphin 0.0036
modern submarine 0.0015
modern surface ship 0.2 to 1.2
inefficient ship hull 3.8

Hope that helps

Cheers

[FlashDrive (Poppy)]

Originally Posted by GeoffW1

Hi Tony,

The formula for water density is hard to find because it is empirical, that is, derived from observations, referenced to a density of 1000 kg/cu metre at 4 deg C.

First get density 'rho' at a certain temperature T:

rho = 1000(1 - (T+288.9414)/(508929.2*(T+68.12963))*(T-3.9863)^2)

then get saline density 'rhos' at a salinity S:

rhos = rho + AS + BS^(3/2) + CS^2

where:

rho and rhos in kg/cu metre
T in deg C
S in g/kg dissolved (seawater is about 35 g/kg)

A, B, C are large coefficients

A = 8.24493E-1 - 4.0899E-3*T + 7.6438E-5*T^2 -8.2467E-7*T^3 + 5.3675E-9*T^4

B = -5.724E-3 + 1.0227E-4*T - 1.6546E-6*T^2

C = 4.8314E-4

Here is the reference you will need:

http://www.earthwardconsulting.com/density.xls

The formula for drag in water (or any fluid including air) is well known, due to Lord Rayleigh:

http://en.wikipedia.org/wiki/Drag_equation

Typical values for drag in water would be:

density rho at 20 deg C : 998 kg/cu metre

'reference area' is aprroximately cross sectional area, so anything you like

'drag coefficient' Cd depends on shape. For:

sphere 0.47
cylinder 0.82
flat plate at 90 degrees 1.17
dolphin 0.0036
modern submarine 0.0015
modern surface ship 0.2 to 1.2
inefficient ship hull 3.8

Hope that helps

Cheers

Errrrr..!! Way over my head .... Sodium + H2o = Salt Water.
Did I impress anybody ....

[GeoffW1 (Geoff)]

I'm fond of saying you can find out ANYTHING here . There is someone who knows, no matter what.

For this I looked up an old textbook, so that was OK. However much of the stuff posted here on other subjects has the same spinout effect on me too

Cheers

[that_guy (Tony)]

Thanks all another questions, is there a formula for calculating visability in certain depth??

[avandonk]

Wearing drag underwater just ruins your dress!

Bert

[GeoffW1 (Geoff)]

Originally Posted by avandonk

Wearing drag underwater just ruins your dress!

Bert

[Barrykgerdes]

Originally Posted by avandonk

Wearing drag underwater just ruins your dress!

Bert

Gee Bert. Can we have a pic.

Barry

[GeoffW1 (Geoff)]

Originally Posted by that_guy

Thanks all another questions, is there a formula for calculating visability in certain depth??

Hi Tony,

There is a well known one for visibility from the surface looking vertically down. This is used to quantify the clarity of the water at a location.

Another formula gives horizontal visibility at shallow depth for various water conditions. This is used to estimate visibility for commercial divers etc. It depends on knowing a factor representing water clarity.

I don't know of a formula to give visibility when underwater, at any given depth, where the light is dimmer.

Would these formula be of any use?

Cheers

[OICURMT]

Originally Posted by that_guy

Thanks all another questions, is there a formula for calculating visability in certain depth??

"Visibility" through water is dependent on its NTU (Nephelometric Turbidity Unit), light source intensity, distance and direction.

NTU's are not calculated, but obtained from observation and lab testing.

I am not aware of any specific equation that "calculates" the distance or visibility through water.