# Water doesn’t fall in inverted half cup

• B
• Kashmir
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.
Kashmir
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?

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.

Juanda, vanhees71 and DaveE

## Why doesn't water fall out of an inverted half cup?

Water doesn't fall out of an inverted half cup due to the surface tension of the water and the air pressure acting on the surface of the water. The cohesive forces between water molecules create a seal that, combined with atmospheric pressure, keeps the water in place.

## What role does air pressure play in keeping the water in the cup?

Air pressure plays a crucial role by exerting force on the surface of the water from the outside. This pressure counteracts the force of gravity trying to pull the water down, thereby keeping the water inside the inverted cup.

## Does the size of the cup affect whether the water will stay in?

Yes, the size of the cup can affect whether the water stays in. A smaller cup has a smaller surface area, which makes it easier for surface tension and air pressure to keep the water in place. Larger cups may not maintain the same effect due to a larger surface area and potentially weaker surface tension relative to the weight of the water.

## Can this phenomenon be demonstrated with other liquids?

This phenomenon can be demonstrated with other liquids that have similar surface tension properties to water. However, liquids with significantly lower surface tension or higher density may not exhibit the same behavior as water does under an inverted half cup.

## What scientific principles explain this behavior?

The primary scientific principles explaining this behavior are surface tension and atmospheric pressure. Surface tension is the cohesive force between liquid molecules, while atmospheric pressure is the force exerted by the weight of the air above the liquid. Together, these forces can counteract gravity and keep the water from falling out of the inverted cup.

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