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# Re: bending of object thrown into pool

• From: John Denker <jsd@AV8N.COM>
• Date: Fri, 13 Aug 2004 14:18:52 -0400

I wrote:

>>The behavior will be verrry complicated. In addition
>>to plain old buoyancy there will be hydrodynamic
>>forces such as lift and drag, which in turn depend on
>>the angle of attack and the sideslip angle etc., not
>>to mention dependence on Reynolds number etc. etc. etc.

Carl E. Mungan wrote:

I did in fact mean the object is *moving*, crossing the boundary from
one fluid to another. But focus on the bending as it crosses the
interface, not the longer term effects of drag and so on. Perhaps

Lift and drag are not "long term" effects ... it seems to me
they are (along with weight and buoyancy) the most important
parts of the story. Lift and drag are simply, by definition,
the two main components of the total fluid-dynamic force.
For definitions, diagrams, and discussion, see:
http://www.av8n.com/how/htm/4forces.html
Other things like pitching moments and "bending" moments
are just higher-order terms in the multipole expansion of
the distribution of forces ... whereas lift and drag have
to do with the total, i.e. the zeroth moment of the
distribution of forces. I don't see much point in worrying
about the higher-order moments until you've got a handle on
the zeroth moment.

-- In general, the motion of an object through air is
verrry complicated.
-- In general, the motion of an object through water is
verrry complicated.
-- The behavior at the air/water interface is even worse;
it is one of the most complicated questions to appear on
phys-l in years. If you want a simple answer, I guarantee
you will be disappointed.

In particular, consider one well-known special case: skipping
stones across the water. That is not going to be described
by anything as simple as Snell's law or anything remotely
similar. It's just plain complicated. FWIW skipping is not
limited to thin flat disks; you can also skip spheres, long
(stick-like) cylinders, and other shapes.