# Exploring Physics: A Deeper Look at Air Pressure

• Hepic
In summary, the conversation discusses the concept of using a straw to hold liquid and how atmospheric pressure and air flow contribute to this phenomenon. The force of air pressure on the surface of the water inside the straw, along with the effects of gravity and surface tension, allow the liquid to be held in the straw. The maximum possible height that this method can hold is approximately 760 mm of mercury or 30 feet of water.

#### Hepic

Check this video.
How physics explains that? I know that it is because of atmospheric air,but I want more specific explanation.

Thanks!

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The air pressure leads to a force on every surface, including the surface of the water inside the straw. Atmospheric pressure is ~10^5 Newtons per square meter, which is probably around 1 Newton for the cross sectional area of a straw. The weight of the water is less than 1 Newton, so the force from atmospheric pressure is sufficient to keep the water suspended in the straw.

it works for exactly the same reason you are able to suck liquid through a straw --- have you thought about that. I mean, if this puzzles you, why are you not puzzled that you can drink soda through a straw?

After the top of the straw is sealed, and as the straw is removed from the water, some of the water flows out the bottom of the straw, expanding and reducing the pressure of the air between the sealed top of the straw and the surface of the water inside the straw, and the pressure differential between the ambient pressure air outside the straw and the reduced pressure air within the straw generates enough upwards force to oppose gravity. The viscosity and the surface tension at the bottom of the straw "seals" the bottom of the straw well enough to prevent air from flowing up through the water inside the straw from below.

okay, people who answered this, tell me this. what is the maximum possible height which is can hold using this method.

oneomega said:
okay, people who answered this, tell me this. what is the maximum possible height which is can hold using this method.

760 mm of mercury or about 30 feet of water.

http://www.thermospokenhere.com/wp/02_tsh/B080___torricelli/barometer.png [Broken]

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## 1. What is air pressure?

Air pressure is the force exerted by the weight of air molecules in the Earth's atmosphere. It is the result of the collisions of these molecules with objects and surfaces.

## 2. How does air pressure affect weather?

Air pressure plays a crucial role in determining weather patterns. High pressure systems typically bring clear and dry weather, while low pressure systems often bring cloudy and rainy weather.

## 3. What tools are used to measure air pressure?

A barometer is the most common tool used to measure air pressure. Other tools include a manometer, aneroid barometer, and digital weather stations.

## 4. What factors can affect air pressure?

Air pressure can be affected by changes in temperature, altitude, and the amount of water vapor in the air. It can also be influenced by large-scale weather patterns and the rotation of the Earth.

## 5. How is air pressure related to altitude?

The higher the altitude, the lower the air pressure. This is because as you go higher in the Earth's atmosphere, there are fewer air molecules above you, resulting in less weight and lower air pressure.