Hydraulic Elevator

In an age-old demonstration, a water-filled bottle with a card over its mouth is inverted, and atmospheric pressure is shown to be greater than the water pressure, for the card is held suspended in place (see Figure 1). The following activity serves as a logical extension of that demonstration. The question is posed: if atmospheric pressure is greater than the water pressure, then the net force on the card should be upward. Why, then, does the card not move upward? Well of course, it doesn’t fit! But what if it did . . . ? {91613_Introduction_Figure_1}