Physics of the fish tank
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Buoyancy
Great article! The only thing I found that needed some further explanation was Buoyancy and why objects seem to weigh less under water than out of water. Hopefully this helps a little:
The cause of the buoyancy of an object in a fuild is caused by a difference in pressures. The bottom of a submerged object is at a greater depth than the top of the object, therefore it has a greater pressure exerted on it by the fluid. The difference of pressures between the top and bottom of the object along with the tendency of pressures to equalize causes an upward force on the object. This upward force opposes the gravitational pull(mg) and causes the object to exert less downward force on the weighing device.
For example:
-A 3' long, 1" diameter rod submerged horizontally is less buoyant than the same rod submerged vertically.
-You are less buoyant lying in bed than you are standing up.
-You can float a bowl made of metal (more dense than water) because the pressure exerted on the top of the bowl by the atmosphere is so much less than the pressure exerted by the surface tension of the water on the bottom of the bowl that the resulting force counteracts gravity's pull. As soon as the water completely covers the bowl when pushed down, the buoyancy provided by the difference in pressures is not great enough to counteract the earth's gravitational pull on it.
So if this happens in air too, why is the measured weight in air more than the measured weight in water? Because pressure increases faster with depth in water than air. The same object has a greater difference in pressure in water than air and appears to weigh less.
I Hope that helps.
Great article! The only thing I found that needed some further explanation was Buoyancy and why objects seem to weigh less under water than out of water. Hopefully this helps a little:
The cause of the buoyancy of an object in a fuild is caused by a difference in pressures. The bottom of a submerged object is at a greater depth than the top of the object, therefore it has a greater pressure exerted on it by the fluid. The difference of pressures between the top and bottom of the object along with the tendency of pressures to equalize causes an upward force on the object. This upward force opposes the gravitational pull(mg) and causes the object to exert less downward force on the weighing device.
For example:
-A 3' long, 1" diameter rod submerged horizontally is less buoyant than the same rod submerged vertically.
-You are less buoyant lying in bed than you are standing up.
-You can float a bowl made of metal (more dense than water) because the pressure exerted on the top of the bowl by the atmosphere is so much less than the pressure exerted by the surface tension of the water on the bottom of the bowl that the resulting force counteracts gravity's pull. As soon as the water completely covers the bowl when pushed down, the buoyancy provided by the difference in pressures is not great enough to counteract the earth's gravitational pull on it.
So if this happens in air too, why is the measured weight in air more than the measured weight in water? Because pressure increases faster with depth in water than air. The same object has a greater difference in pressure in water than air and appears to weigh less.
I Hope that helps.
ummm, 14.7 PSI (or 29.92 inches) is not a normal day at any altitude. 29.92 inches (I use inches because I'm used to it) is actually the standard pressure at sea level when the air is at 59 degrees F, I forget about humidity. Well now this is at sea level, and few people are at sea level. As altitude increases, pressure decreases as well as temprature. Just thought I would throw that out.
It has less to do with pressure and more to to with density of the fluid the object is submerged in and the density of the object itself. Density = mass / volume. When you submerge an object it in a fluid the object displaces a volume of the fluid equal to the volume of the object. An object will appear lighter by an amount equal to the mass of the volume of fluid the object is displacing. For example near sea level air has a density of about 0.0012 grams per cubic centimeter, while water has a density of about 1.0 grams per cubic centimeter. The density of each is fluid is effected by pressure, air much more so than water because air is much more compressable.
Now lets say you have a 1 cubic centimeter piece of aluminum with a density of 2.7 grams per cubic centimeter. (A 1cc object keeps the math simple in the examples). In a complete vacuum that piece of aluminum will have a true mass of 2.7 grams. Submerged in air with a density of 0.0012 g/cc this same piece of aluminum will have an apparent mass of 2.6988 grams (2.7 - 0.0012 = 2.6988). Submerged in water with a density of 1.0 g/cc this same piece of aluminum will have an apparent mass of 1.7 g/cc (2.7 - 1.0 = 1.7). Now lets suppose you submerge this aluminum object in mercury with a density of about 13g/cc. In this last example the aluminum object would have and apparent mass of -10.3 grams (2.7 - 13 = -10.3) and would float because the mercury is more dense and gravity would have a stronger pull on the mercury than the aluminum.
The reason that a hollow metal object can float on water is because it's density (mass / volume) is lower than the density of water.
The density of matter varies with it's temperature & its rate of thermal expansion, and with the pressure exerted on it along with it's compressability both of which are effectively changing it's volume and these properties are different from one material to the next while it's true mass remains constant for as long as it is still made up of the same number and type of atoms.
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Further discussion to be placed here:
http://www.aquariacentral.com/forums/showthread.php?p=1495307#post1495307
http://www.aquariacentral.com/forums/showthread.php?p=1495307#post1495307
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