My son just asked me at dinner and I do not have an exact answer.
In: 7
To get the water in the straw, the suction force you apply had to overcome the gravitational force pulling the water down (the obvious bit). When the suction stops (the one end of the straw is opened), gravity draws the water down again. The downward movement of the water in the straw creates a slight low pressure as the water flows down which sucks in air in the wake of the dropping water (the air replaces the water in the tube, almost like what happens when you drain a big bottle of liquid). However, when you block the top of the straw, you’re removing the flow of air from the equation. Now, the gravitational force is still pulling down on the water in the straw, but since the low pressure that forms as the water is pulled down cant be equalised by the air through the top, a vacuum forms between the top of the water and the blocked straw opening, which is equal to the gravitational force downwards (which is why the water in the straw stays at a constant level after adjusting).
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