Of course this means weighing any living animal by displacement of water will be inexact to the tune of their lung volume (including fish who vary their buoyancy with their swim bladder).
I don't think this is the case for things that are floating, which includes fish. The force pushing them down precisely balances the force pushing them up - otherwise they would sink or float on up out of the water. And the amount of water pushed out of the way by the thing must weigh the same as the thing - otherwise there would be a net force pushing it up or pushing it down.
It totally doesn't work for things that are rising or sinking (forces are not balanced), or for things that have sunk, because there the forces are balanced at least in part by the ground pushing up.
The adjusting-your-height-in-the-water business - whether humans breathing in or out, or fish with swim bladders - works on this principle. If you are static at one height in the water, and then breath in/expand your swim bladder, you displace more water, so there is a net upwards force (because your weight hasn't increased). This force will act to push you upwards until you displace less water so it balances your weight. If you are a human, this happens mostly by more of you being out of the water; if you are a fish, this happens mostly by the water being less dense the higher you go. Water is really pretty incompressible - indeed, to a first approximation in most practical contexts, you can't compress it at all. So you only need to make relatively small adjustments to your volume to have neutral buoyancy at pretty wide range of depths. Which makes swim bladders very practical - and submarine ballast tanks, and scuba buoyancy control devices), and so on. In fact, with the latter, the main size requirement is to be big enough to keep you well out of the water at the surface.
Want another question now :-)
There're venta's one about getting a giraffe across London waiting for you upthread. :-)
Of course this means weighing any living animal by displacement of water will be inexact to the tune of their lung volume (including fish who vary their buoyancy with their swim bladder).
I don't think this is the case for things that are floating, which includes fish. The force pushing them down precisely balances the force pushing them up - otherwise they would sink or float on up out of the water. And the amount of water pushed out of the way by the thing must weigh the same as the thing - otherwise there would be a net force pushing it up or pushing it down.
It totally doesn't work for things that are rising or sinking (forces are not balanced), or for things that have sunk, because there the forces are balanced at least in part by the ground pushing up.
The adjusting-your-height-in-the-water business - whether humans breathing in or out, or fish with swim bladders - works on this principle. If you are static at one height in the water, and then breath in/expand your swim bladder, you displace more water, so there is a net upwards force (because your weight hasn't increased). This force will act to push you upwards until you displace less water so it balances your weight. If you are a human, this happens mostly by more of you being out of the water; if you are a fish, this happens mostly by the water being less dense the higher you go. Water is really pretty incompressible - indeed, to a first approximation in most practical contexts, you can't compress it at all. So you only need to make relatively small adjustments to your volume to have neutral buoyancy at pretty wide range of depths. Which makes swim bladders very practical - and submarine ballast tanks, and scuba buoyancy control devices), and so on. In fact, with the latter, the main size requirement is to be big enough to keep you well out of the water at the surface.
Want another question now :-)
There're venta's one about getting a giraffe across London waiting for you upthread. :-)
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