Question

A glass is filled into its brim and a thick hand card is placed on top of it. Now, keeping the card tightly closed and pressed with palm, the glass full of water is inverted. Palm is gently removed from the card. It is observed that the card doesn’t fall off. Why does this happen?

Answer 1

The air that is stuck inside the glass of water exerts some amount of air pressure on the card. There’s also some amount of pressure exerted on the outside of the card. Compare the pressures exerted in and outside of the glass and card.

Complete answer:

When we remove our hand slowly from the card, we will observe that the card doesn’t fall off the glass because the air stuck in between the plank or card and the glass of water exerts some amount of pressure. As the air outside the setup exerts more pressure on the card than the pressure exerted due to the air inside it, the card won’t fall off. The pressure exerted by air upwards is more than the pressure exerted by the air downwards.

Additional information:
Air pressure is the weight of the air molecules pressing down the earth. The pressure of the air molecules changes as you move towards height into the atmosphere. Pressure is highest at the sea level as the density of the air molecules is greatest at the point. We use barometers to measure the pressure of the air during weather forecasts and weather patterns. We measure the pressure in either inches or centimetres.

Note:
Air pressure drops usually in the case of compressed air. This is because the restriction in the system when compressed will cause resistance to air flow and because pressure drops. Thus, usually air separators or lubricants, after coolers, moisture separators etc. Are the main reasons that cause the pressure to drop significantly.