(First a correction - in my previous area calculations I was squaring
the diameter instead of the radius. What I said would apply to a 12 inch
funnel. The conclusion does not change.)
I don't know. I can't get this to happen, using a variety of pot and jar
lids and metal bowls held upside down as a pot of water is filled.
Everything wants to pop up when I let go. Are you sure that the air
didn't escape as you were filling it? You saw no bubbles? Were you
pushing on the center of the lid as you held it down, deforming it a little?
A really heavy lid with a very small volume of air under it might hold
itself down.
Steel will normally sink in water, but when it encloses a volume of air,
the water pressure differential should be transmitted to the air and
push the whole thing up. When you want to lift a sunken ship, you fill
it with air.
Is there a soft plastic rim on the lid that might be forming a seal
against the bottom? Is your classroom in free-fall? Now I'm venturing
into the "look like a fool when your explanation is completely wrong"
zone, but I'm going for the seal explanation. The surface tension of the
water plus the increase in air pressure from pressing down on the center
of the lid keeps the water from getting under the lid and exerting a
vertical force. Can you try holding the lid down from the edges when
filling the container?
For what it is worth, I placed a large shallow screw-on top (like the
lid to a can of baby formula but deeper) on the bottom of a larger
container. I then filled the container with water, holding down the lid
while it filled. I then let go.
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Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@po-box.esu.eduhttp://www.esu.edu/~bbq <http://www.esu.edu/%7Ebbq>