The toothpaste experiment has been exposed partially by JD who showed that
toothpaste does act as a glue. But there is a big flaw in the toothpaste
experiment. As you lower the cup, the air inside it compresses which
produces an increased upward pressure on the cup. This is not the same as
the glue or my proposed vacuum seal, because this upward pressure does not
go away. The card on the box is a variant of my vacuum seal, but
unfortunately the card would bend as you lower it.
How about a variant on the vacuum seal with an open bottom cup which has an
o ring seal. On the top of the cup have a flexible tube. As you lower the
cup to the bottom blow into the tube to keep water out of the cup. Then
firmly press the cup to the bottom sealing out the water. At that point it
should stick to the bottom. Notice that the tube allows you to have 1 atm
of air in the cup as you would in a rigid massless box. This might actually
be doable. If the cup stays down, you have proof that there is a
compressive force. And if somebody designs this, PASCO might even be
willing to produce it. They might even pay a small sum for the rights, so
you can maybe buy a cheap bottle of wine in celebration.
So the toothpaste is really acting as a glue, and it is under tension.
Actually we don't need to do this because it is completely analogous to a
suction cup in air, only it is a water suction cup. Suction cups in air are
"attempting" to resume their normal curved shape, but the air pressure on
them is pushing them into the surface. In a similar way an empty box would
be pressed down by the water pressure.
So now, the last thought is just do it with a normal rubber suction cup.
They float and are buoyant, but if pressed to the bottom they should stay,
demonstrating that there really is a compressive force. Does anyone doubt
that a suction cup will work underwater? This now provides an anchor for
the series of analogies which bridge over to the target of a glued empty
box. At that point, we have an anchor and bridging analogy which can be
used with students.