Re: Buoyancy gedanken
- From: "Leo Lichtman" <l.lichtman@xxxxxxxxxxxxxxxx>
- Date: Tue, 05 Jun 2007 20:01:59 GMT
<spamTHISbrp@xxxxxxxxx> wrote: in message
news:1181069323.278859.32430@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Not really OT givien the recent thread.^^^^^^^^^^^^^^^^^
Buoyancy gedanken
Take a 3 inch inside-diameter tube, one end is open, the other end is
closed with a cap with a rod going through it, and this terminates in
a piston that seals watertight to the walls of the cylinder.
The purpose of the piston/rod is to form a moveable bottom for the
cylinder.
Think of it as a hydraulic cylinder with the piston and shaft still in
place, but the other end is cut off.
Let the piston have a valve in it so that water can be drained through
to the bottom of the cylinder, and the bottom of the cylinder also has
a drain valve.
The system starts with the piston at the bottom of the cylinder, the
piston valve is open, the cylinder valve is closed.
Whatever cylinder volume exists below the piston is filled with water,
and the rod seals watertight to the cap.
Further assume the rod is infinitely thin, so the volume of the rod
inside the cylinder does not increase as the piston is pushed up into
the cylinder.
(Or, just forget the rod, and assume the piston moves up and down the
bore via magic, magnetism, etc.)
Water fills the cylinder (held open end up) to within say 1 inch of
the top.
An item floats on top of the water, not fully submerged. The moveable
bottom is, say, one foot from the bottom of the floating object.
Lets make it easy and say the object is cylindrical and is only
slighty smaller than the bore of the cylinder.
Keep the piston valve open, keep the cylinder valve closed. Piston
moves up, water level does not change since the water above the piston
moves to below the piston via the valve.
(clip) Bring the piston up to, say, 0.125 inches below the object.
Is the object still flaoting?
Has its position in the cylinder changed?
It floats to a depth at which it displaces its own weight of water.
Depending on its shape, that could be more or less than 1/8". If it is
more, then it is resting on the piston--not floating.
^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^
Assume (clip) the weight of the water above the piston is less than that
of the floating object.
Is this even possible?
Visualize a piston shaped so it fits around the floating object nicely, It
would then be possible for the object to float in a tiny amount of water,
and still displace its own weight.
^^^^^^^^^^^^^^^^^^^^
Close the valve in the piston.
^^^^^^^^^^^^^^^^^^^^
Does the object move?
No. The object still floats to a depth at which it displaces its own
weight.
^^^^^^^^^^^^^^^^^^^^
Drain the water below the piston.
^^^^^^^^^^^^^^^^^^^^
Does the object move?
No. Once the valve is closed, events below the piston have no effect on the
water or what floats in it. The object still floats to a depth at which it
displaces its own weight.
Why do you ask? Is this part of your high school physics homework?
.
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