When a glass that is fully filled with water and covered by thin paper is inverted the water does not fall because (TOPIC RELATED - PRESSURE)
Answers
Answer:
Fill a glass part way with water. Turn it upside-down. You now have water on the floor. Why did you listen to me?
Pour water in the same glass again. Put an index card over the mouth of the glass and press the palm of your hand on the index card, pressing the card against the rim of the glass and depressing it slightly into the glass in the center (this part is very important). While your hand is on the index card over the mouth of the glass, invert the glass and slowly take your hand away. If you hold the glass steady and level, the water should remain in the glass (Fig. 1).
the explanation
Why doesn't the water fall out of the glass with the index card?
The answer has to do with air pressure. Any object in air is subject to pressure from air molecules colliding with it. At sea level, the mean air pressure is one "atmosphere" (=101,325 Pascals in standard metric units). This air pressure is pushing up on the card from below, while the water is pushing down on the card from above. The force on the card is just the pressure times the area over which the pressure is applied; that's the definition of pressure.
Force=Pressure×Area
If you've done the trick correctly, the force from the air below exactly counteracts the force from the water above, and the card stays in place.