[Phys-L] Re: buoyancy

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 09:53 AM 2/1/2005, you wrote:
> Objects float in water because the weight of the displaced water equals
> the object's weight. I put a beaker of water on a scale and ask my
> students what will happen to the reading if I dip my hand in. Most say it
> will stay the same, but are surprised to find the reading actually
> increases -- by the weight of displaced water my finger pushes away. I ask
> if a live fish were placed in the water, would the reading change? Yes --
> by the weight of displaced water once again. But what if the fish dies and
> goes to the bottom of the beaker? The scale reading should reflect this.
> It should increase by the actual weight of the fish. But does the weight
> of displaced water matter?
>
> I tried this with a golf ball, each weighed (massed) separately. When I
> dropped the ball in, the scale only increased by the ball's weight. There
> is still a buoyant force upward, and this balances the (downward) weight
> of displaced water. So the reading should only increase by the object's
> weight. Is this correct reasoning?


It's helpful to review four buoyancy cases:
1) It floats
2) it sinks
3) It would sink, but is suspended by a thread or some such.
4) it hangs in the middle of the water.

If there is no other support for the wetware, all its weight shows
  up on the container.  Case 1) 2) 4)

If there is some external support for the wetware, part of its weight
  is taken by that external support - the cotton thread in the usual
  experimental rig like case 3) but could be 1) and 2) as well.


If the wetware floats, its submerged volume displaces a volume of
  water weighing the same as its weight, which is less than its total volume.
If the wetware sinks, its submerged volume displaces a volume
of water weighing less than the wetware's weight.
If the wetware would sink, but is supported by a thread in the water,
its submerged volume displaces an amount of water weighing less than
  the wetware's weight,    and the thread supports the remainder
  of its weight.
If the wetware is neutrally buoyant, its submerged volume
displaces an amount of water equal to its weight.

Fish arrange to be neutrally buoyant in two ways:
- a swim bladder of gas
or
- an enlarged liver full of oil, accounting for as much as
  25% of the fish's mass. (The shark approach - though some
shark are said to swim perpetually to avoid sinking)



Brian Whatcott    Altus OK    Eureka!