Why does cardboard stick

Best performed over a teacher's head. Fill a glass part way with water. Turn it upside-down. You now have water on the floor. Why did you listen to me? Pour water in the same glass again. Put an index card over the mouth of the glass and press the palm of your hand on the index card, pressing the card against the rim of the glass and depressing it slightly into the glass in the center this part is very important.

While your hand is on the index card over the mouth of the glass, invert the glass and slowly take your hand away. If you hold the glass steady and level, the water should remain in the glass Fig. The answer has to do with air pressure. Any object in air is subject to pressure from air molecules colliding with it.

This air pressure is pushing up on the card from below, while the water is pushing down on the card from above. The force on the card is just the pressure times the area over which the pressure is applied; that's the definition of pressure. The details of this delicate balance are more easily understood by looking at the forces on the water, rather than on the card see Figure 2.

The card transfers the force of the air pressure upward to the water, so there is a pressure of almost 1 one atmosphere pushing up on the water from below.

Of course there is also pressure from the air inside the glass pushing down on the water from above. The air inside the glass was originally at one atmosphere of pressure when you put the card over it, but when you inverted the glass and removed your hand, the water moved downward a very slight amount perhaps making the card sag ever so slightly , thereby increasing the volume allotted to the air.

As the air expands to fill this increased volume, several things happen at once. There are a couple places where pressure is pushing against the card. First the atmospheric pressure or the tiny air molecules all around us are randomly colliding with the bottom of the note card. There are more air molecules pushing up against the bottom of the note card, creating a higher pressure area compared with the lower pressure area inside the air pocket in the glass.

This is a great explanation of why this works, but there is more. Write down your hypothesis prediction and then follow the steps below. Do you know why this happens? Find out the answer in the how does this experiment work section below. Upside Down Water Glass Video.

The reason this experiment works is because of air pressure! Air pressure is the weight of a column of air pushing down on an area. While we cannot feel it, the air is heavy!

The weight of the air pushing down on all objects on Earth is the same as the combined weight of three cars! In this experiment, the air pushing up from underneath the paper is strong enough to overcome the weight of the water pushing down on the paper. That's where surface tension works. The contact angle of water in water-glass interface being acute, the surface of water in contact with the air is bulged a bit inwards.

As a result, the surface film is pulled in the outward direction due to the reaction forces from the glass plates due to the surface tension of water pulling down on them much like in a capillary. Therefore an excess of pressure pushes down on the upper plate from the up which makes the separation difficult. Plus the adhesive forces between the water and glass molecules make them stick to the water layer, which further adds to the diffculty.

In case the plates are left undisturbed, the water surface at the air-water interface actually bulges outward for obvious reasons, well, actually this time the excess pressure from water balances the upper plate's weight Ok, so don't get confused.

Ok, so now what happens in the water and cardboard case is that the cardboard slightly moves down allowing for the same tiny air-water interface near the edges. To maintain an equilibrium the pressure on the inside drops down below atmospheric pressure therefore creating a difference of pressures sufficient enough to hold the cardboard's weight.

Plus the adhesive forces also add to the stability. Ok so now you might ask how, because the pressure on the inside must be more than the atmospheric pressure because of the air already present in the empty space above the half filled glass.

What happens in this case is that because the cardboard goes a bit down the volume of the empty space increases. Initially the pressure of air in the empty space was equal to the atmospheric pressure, but now an increase in volume lowers it down.

In some cases you can also see a bit of the water getting out of the system during your upside down turn. This is to ensure that the volume of the empty space above gets high enough and therefore the pressure gets low enough so that the balance can be maintained. In a simple words, I think everything takes up space, so if you prevent the water from falling out, you succeed in preventing the air from getting in. By this way, nothing will change, everything will stay the same.

Atmospheric pressure is caused by air gravity and air molecules movement, air pressure in the half filled upside-down cup is lower than the air pressure outside due to less gravity, so the air pressure can still hold the water in half filled cup. The plate will be held as long as we are breaking the force, which is provided by the surafce energy of water with contacting medium - air in the current case. As Mike said, in the answers In vaccuum assuming water not boilling!!!

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