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[Phys-L] Re: Buoyancy question

On Wed, 8 Dec 2004 22:17:54 -0600, Brian Whatcott <betwys1@SBCGLOBAL.NET> wrote:

If I place a sewing needle on a piece of tissue paper and
float it on clean water, the paper waterlogs and sinks,
and the needle floats.
If I examine the surface of the water I see that it is
depressed under the needle. The depression curves up
smoothly to the surface.
Though the water surface layer is supportive, I wonder if the total
depression displaces as much water weight as the needle weighs?


No, it has to occur before there is that much water displaced.

When the surface is stretched, its PE increases, at an increasing rate
(w.r.t. displacement), going until it reaches a maximum level - at which
point it breaks. Let's assume we don't get there - otherwise we don't have
a problem to consider.
As the needle settles onto the water, its PE decreases, but that of the
displaced water increases, at an increasing rate (displaced water is moving
over a greater vertical distance). The net effect is one of decrease, at a
decreasing rate.

When the two rates match (so add to zero), we have a local equilibrium and
that is where the needle floats.

Suppose though that the combined grav PE fell more rapidly than the PE of
the surface rose, even up to the point where the needle was entirely below
the water "level". Suppose further that the surface does not close in on
itself - like dropping a cannonball onto a loose sheet of polythene - that
is trivial, it's going to sink!.

Eventually, the rate at which the PE of the needle falls is going to be
matched by the rate at which the grav PE of the water increases - that would
be the normal floating point, where the needle "displaces its own weight".

However, while it is true that at this point the rate of change of the two
gravitational PEs balance, the PE of the surface is still increasing. The
total PE is not a minimum, so it cannot be an equilibrium point. The needle
cannot stay there.

At some point before, the PE loss of the needle would have balanced with the
PE gain of the water and the PE gain of the surface - that is the
equilibrium point, and it must occur before the needle displaces its own weight.
This is, of course, assuming that the surface behaves in an elastic manner,
but if not, we are back to the trivial problem of cannonballs and polythene
again.


Brian W


At 02:41 PM 12/8/2004, you wrote:
If you gently place an ordinary needle sideways on the surface of the
water it will float. I did this back in elementary school as I
recall. I think you have to lower it with a fork, and be extremely
gentle. The surface tension will hold it up. But if you put a drop
of detergent into the water, it sinks. This is obviously not a
buoyancy effect, as it will sink if you just drop it in or put it in
point first. It is a fun little amazing experiment.

John M. Clement
Houston, TX


I'm having trouble understanding Clement's response.

On Tue, 07 Dec 2004 23:30:53 -0600 John Clement <clement@HAL-PC.ORG>
writes:
I believe he is referring to the fact that a needle if gently placed
on the surface floats. This is a surface tension effect and not a
buoyancy
effect. Of course if you add a drop of detergent it doesn't work.

John M. Clement
Houston, TX


Greetings everyone!

I have a question pertaining to buoyancy phenomenon:

What's the condition that a piece of iron released gently on the
surface of water sink? Ignore surface tension.

Thanks,

Hasan Fakhruddin
Instructor of Physics
The Indiana Academy for Science, Mathematics, and Humanities
BSU
Muncie, IN 47306
E-mail: hfakhrud@bsu.edu




Herb Gottlieb from New York City
A friendly place to live and visit


Brian Whatcott Altus OK Eureka!