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Why pulling a paper too quickly from underneath a filled cup won’t make the liquid flow?
According to physics, this question deals with momentum and time. Does the water remain fixed because less time has been taken and greater force has been exerted or have we increased the momentum?
Same reason as when you take a straw in a cup filled with water, stick your finger over the top and then pull the straw out of the cup. The water will stay in the straw
Why?
Pressure
Basically there’s air pressure acting at both ends of the body of fluid, one from the air at the bottom of the water and one at the top of the water between the water and the cup. If the water were to fall out, the volume of air at the top of the cup would become larger – but only the same amount of air is there (i.e. the mass of air hasn’t changed). Thus, the air at the top of the cup needs to expand which decreases the pressure.
Thus, the air at the top of the cup will be exerting less force than that from the air at the bottom of the water (or outside the cup). This creates an overall force that pushes the water back into the cup, until it comes to an equilibrium (due to the weight of the water).
Air pressure is very strong – look up Magdeburg hemispheres if you want to see just how strong it is.
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