Why does water stay in the straw when a finger is pressed over one end? How does sealing off the one end make the pressure less?
When you fill a straw with water and then seal one end with your finger, you can then hold the straw vertically without any water falling out of the straw. That’s because the air pressure above the column of water decreases until the upward force caused by the unbalanced pressure at the top and bottom of the water column is exactly equal to the weight of the water column. The drop in pressure above the water column occurs because the water initial does fall downward. When you first tip the straw from horizontal to vertical, the air pressures above and below the water column are equal and there is no pressure force to opposite the weight of the water. The water begins to fall. As it does, it creates a relatively empty region above the water column and below your finger. The air molecules in that region become sparser and their pressure decreases as a result. The water descends just far enough to lower the pressure inside that trapped air region until the pressure force balances the water’s weight. Actually, the water column bounces up and down briefly, just like a weight at the end of a spring or a person at the end of a bungee cord. But after a second or so, the water column just hangs there motionless in the straw, supported against gravity by the pressure imbalance. If air could work its way through the water column and enter the trapped region between the water column and your finger, the water column would be able to descend further. But the straw is so narrow and the water sticks to tightly to itself (a phenomenon called surface tension) that it prevents air bubbles from working their way up the straw.