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Surface Tension and Floating
Name: Lydia D.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002
Question:
We did an experiment in class today that had to do with
water surface tension: You put a cork in a glass half full with water and
it floats to the side, but stays in the middle of a full glass.
Supposedly this is because the surface tension of a full glass produces a
convex surface and the cork floats on top, and adhesion makes it stick to
the half-full glass, making a concave surface, and the cork floats where
the water rises (are you following this?). However, logic tells me that
the cork should obey gravity and float at the lowest point of the water.
My teacher says that things actually float at the highest point of water,
such as waves, due to some complex physics principle he wouldn't go into.
Would you please explain this to me? Thank you very much.
Replies:
Lydia,
Any object feels an upward force called "buoyancy" from the fluid it sets
in. Objects in water feel the upward buoyancy force from the water. You
feel an upward buoyancy force from the air, as does a helium balloon. Every
object also feels a downward force from gravity: every object feels its
weight. When an object weighs more than the upward buoyancy, the object
sinks. When weight is less than buoyancy, the object floats.
A floating object rises until only a small part of it is in the water. This
makes less of the object be in the water, so that upward buoyancy force is
smaller. The object rises until the buoyancy and weight are equal. Because
that buoyancy force from the water is upward, the cork moves upward in the
water until it reaches the top.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
I do not buy the explanation. Here is why. Surface tension of water is
EXTREMELY sensitive to many factors. Even microgram amounts of the
appropriate surface active agent (soaps for example) can cause it to
decrease from its "normal" value of ~72 ergs/cm^2 and the shape of the
meniscus is a very complicated function of the surface tension. In any case,
for all practical size glasses, the shape of the surface at the center is
essentially flat in both cases you tried. The cork does not even "know" there
is a boundary, much less know its shape.
You will need to conduct numerous trials to eliminate a large number of
alternative explanations other than the one proposed. The following come to
mind, but this list is by no means complete: 1. Is the floating cork
protected from ALL drafts of air? If not, any small air movement -- it may
not even be strong enough to feel the draft -- will push the cork toward the
edge of the glass. In the half filled glass, the walls of the glass shield
the cork from small drafts. 2. What if you use a rectangular or elliptical
glass. If the surface shape argument is correct the cork should always drift
toward the "narrow" directions since they will have the greatest curvature.
3. What happens if you add a surfactant, like ordinary dish soap. This will
greatly reduce the surface tension from about 72 ergs/cm^2 to 20-30
ergs/cm^2, and the shape of the meniscus will be concave upward and the cork
should always stay centered in the glass. 4. How long did you wait before
placing the cork in the water? Eddy currents can be quite persistent and
would tend to remain longer if the glass is full. 5. How many times did you
repeat each experiment? You should do each experiment 20-50 times to be sure
you are not seeing some statistical fluctuations, rather than an actual
effect. 6. Is there different behavior for a "dry" cork vs. a "wet" cork?
The bottom line is: In experimental design jargon, you have an experiment
that has a lot of hidden variables you do not even know about and hence
cannot eliminate) and confounding variables (variables that interact with
other variables that obscure any cause/effect).
An example in your experiment is air movement and depth of the water in the
glass. That MAY be going on, and be the dominant effect while you are
measuring whether the cork moves to the side of the glass. So you think you
have found the CAUSE but in fact something else entirely is going on.
Your experiment is a good one for learning about experimental design.
Vince Calder
How about water obeying gravity and moving to the lowest point it is
physically allowed to move to? The energy it takes to put a volume of
water at the top of the convex meniscus is greater, obviously, than the
energy required to put it at the bottom of the meniscus. The same is true
for the cork, of course, but the density of cork is less than the density
of water, so if a cork is
displacing some water at the bottom of the meniscus, the system can lose
energy by swapping the submerged volume of cork for an equal volume of
water from the top of the meniscus.
Tim Mooney
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