[At' Pressure] Glass of water held upsidedown

In summary, when turning a glass of water upside down with a seal on top, the air pressure inside the glass is greater than the weight of the water, causing the seal to stay in place. This is due to gravity pulling the water down and creating a vacuum at the top, but air pressure resisting this and pushing the water back up. This does not occur when the glass is right-side-up because gravity is holding the water down in the bottom.
  • #1
BitterX
36
0
Hey,
I'm sure you are all familiar with the experiment where you take a glass of water , put a some seal on it and then turn it upside down.
Due to the (atmospheric pressure) - (the air pressure in the glass) exerting a force on the seal upwards greater than the [mg] of the water, the seal stays in place.

here's a demonstration - physicscentral.com

What I don't understand is why there's a pressure difference in the first place.
As far as I know - if I seal the cup then the air pressure is the same there as it is outside (with minor difference - ρgh ) what makes it change when I turn it upside down?

Thanks.
 
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  • #2
BitterX said:
What I don't understand is why there's a pressure difference in the first place.
As far as I know - if I seal the cup then the air pressure is the same there as it is outside (with minor difference - ρgh ) what makes it change when I turn it upside down?

The pressure difference develops because gravity is pulling the mass of the water down in the inverted glass, trying to open up a bubble of vacuum at the top. Air pressure resists the formation of that bubble, pushing the water back up against the force of gravity.

This doesn't happen when the glass is right-side-up, because then gravity is holding the water down in the bottom of the glass.
 

1. How does the water stay inside the glass when it is held upside down?

The water stays inside the glass due to atmospheric pressure. As the glass is turned upside down, the air inside the glass creates pressure that is equal to the atmospheric pressure outside the glass, keeping the water from falling out.

2. What happens to the water if the glass is held upside down for a long time?

If the glass is held upside down for a long time, the water will eventually start to leak out. This is because the air inside the glass will slowly escape, causing the pressure to decrease and allowing the water to fall out.

3. Why does the water not spill out when the glass is held upside down in space?

In space, there is no atmospheric pressure to keep the water inside the glass. However, the surface tension of the water, which is caused by the attraction between water molecules, is strong enough to hold the water inside the glass even without the presence of atmospheric pressure.

4. Does the size or shape of the glass affect the water staying inside when held upside down?

Yes, the size and shape of the glass can affect the water staying inside when held upside down. A wider glass with a larger surface area will have more contact with the surrounding air, creating a stronger pressure that can hold more water. A taller glass may have a weaker pressure and can hold less water before it starts to leak out.

5. Can any liquid be held inside a glass when it is held upside down?

No, not all liquids can be held inside a glass when it is held upside down. The liquid must have surface tension and be able to create a seal with the glass in order to stay inside. For example, oil or alcohol may not have enough surface tension to create a seal and would leak out when the glass is held upside down.

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