Why water in cup suspends when placed upside down in water?

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In summary: Bar = 1 atmosphere = 10 m of water.In summary, the reason why water in a cup does not leak out until the opening of the cup reaches the surface of the water is due to the weight of the liquid outside the cup pressing down on it. This pressure is equivalent to 10 km of atmosphere and is known as the Toricelli effect. In order for the cup to show a vacuum, it would have to be over 10 m high. This can be demonstrated using a garden hose in a 4 storey stairwell. The apparent vacuum at the top of the hose is actually very low pressure water vapor, which is equal to 0.023 Bar at 20 degrees Celsius.
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henry wang

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Why water in the cup doesn't leak out until the opening of the up reaches the surface of water?
Is it because the weight of the water in the cup tries to create vacuum on top of the cup, which exert negative pressure on the water?
 
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  • #2
No. Because 10 km of atmosphere press down on the liquid outside the cup. Read all about it under Toricelli (who did the experiment with a tube and mercury and discovered that 10 km of atmosphere weighed as much as 76 cm of mercury). So before your cup actually shows a vacuum, it would have to be over 10 m high :smile: .
 
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henry wang said:
Is it because the weight of the water in the cup tries to create vacuum on top of the cup, which exert negative pressure on the water?

A vacuum would be zero pressure. There is no such thing as a negative absolute pressure (eg less than a vacuum).
 
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  • #5
BvU said:
So before your cup actually shows a vacuum, it would have to be over 10 m high :smile: .
You could do this with a (preferably transparent) garden hose in a 4 storey stairwell. Have you seen this described anywhere? I guess the water would boil as you pulled the hose above the 10 m level.
 
  • #6
Google water barometer.
Boiling is a big word for what happens at the 10 m level, but you are right: the apparent vacuum at the top is in fact very low pressure water vapour (0.023 Bar at 20 C) so a very small amount of water has to evaporate
 

1. Why does the water in the cup stay suspended when placed upside down in water?

The water in the cup stays suspended due to the force of air pressure pushing up on the surface of the water in the cup. This force is strong enough to counteract the weight of the water and keep it from falling out of the cup.

2. Does the temperature of the water affect the suspension of the cup?

Yes, the temperature of the water can affect the suspension of the cup. Colder water is denser and therefore exerts a greater force on the cup, making it easier for the cup to stay suspended. Warmer water is less dense and may not provide enough force to keep the cup suspended.

3. How does the shape and size of the cup impact its suspension?

The shape and size of the cup can impact its suspension in water. A wider and more shallow cup will have a larger surface area, allowing for more air pressure to act on the water and keep it suspended. A taller and narrower cup may not have enough surface area to counteract the weight of the water and may fall out of the water.

4. Will the type of liquid used in the cup affect its suspension?

Yes, the type of liquid used in the cup can affect its suspension. Different liquids have different densities, and denser liquids will exert a greater force on the cup, making it easier for the cup to stay suspended. Lighter liquids may not provide enough force to keep the cup suspended.

5. Can the suspension of the cup be affected by external factors?

Yes, external factors such as atmospheric pressure, altitude, and humidity can affect the suspension of the cup. Higher atmospheric pressure, lower altitude, and higher humidity can increase the force of air pressure and make it easier for the cup to stay suspended. Lower atmospheric pressure, higher altitude, and lower humidity may decrease the force of air pressure and make it more difficult for the cup to stay suspended.

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