Water doesn’t fall in inverted half cup

In summary, Brian Cox demonstrated on a show about air pressure that when a half-filled cup of water is flipped upside down, the paper on top of the cup does not drop. This is due to a negative pressure developing inside the glass, caused by the paper's deformation and the hydrophilic nature of both the paper and glass. To test this explanation, two possible experiments could be conducted.
  • #1
Kashmir
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I just happen to see a show about air pressure host by Brian Cox. As the link:https://i.stack.imgur.com/vfZlI.jpg shows, he flipped a half-filled water cup upside down then the paper on cup doesn't drop:
It puzzled me, because the cup is half-filled, the pressure shouldn't balance since inside water pressure+atmosphere pressure should be bigger than outside atmosphere pressure only.

What’s the explanation?
 
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  • #2
What does the film say?
The paper deformation will allow a negative pressure to develop inside the glass. The 5 cm of water in the glass will produce a gauge pressure of ~.005atm (10 m is one atm) so the air volume will need to expand by .5% to create that suction. In addition both paper and glass are hydrophyllic and the surface tension at the boundary will tend to retain the paper in place. Apparently the combination is enough.
Can you devise an experiment to test this explanation? I can think of two different ones.
 
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1. Why doesn't water fall in an inverted half cup?

Water doesn't fall in an inverted half cup because of the principles of surface tension and adhesion. Surface tension is the cohesive force that holds water molecules together, creating a "skin" on the surface of the water. Adhesion is the force that causes water molecules to stick to other surfaces, such as the sides of a cup. When a half cup is inverted, the surface tension and adhesion forces work together to keep the water from falling out.

2. How does surface tension and adhesion prevent water from falling in an inverted half cup?

Surface tension and adhesion work together to create a strong bond between the water molecules and the sides of the cup. This bond is strong enough to overcome the force of gravity pulling the water down, keeping it in the cup even when it is inverted.

3. Can any liquid be held in an inverted half cup?

Yes, any liquid that exhibits surface tension and adhesion can be held in an inverted half cup. However, the strength of these forces may vary depending on the liquid, so some liquids may not stay in the cup as well as others.

4. What happens if the half cup is not completely inverted?

If the half cup is not completely inverted, the surface tension and adhesion forces may not be strong enough to keep the water from falling out. This can result in the water spilling out of the cup or dripping down the sides.

5. Are there any other factors that affect whether water falls in an inverted half cup?

Yes, temperature and the material of the cup can also affect the surface tension and adhesion forces, which in turn can affect whether water falls in an inverted half cup. For example, a cup made of a material with low adhesion, such as Teflon, may not be able to hold water in an inverted position as well as a cup made of a material with high adhesion, such as glass.

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