Air Pressure

We have seen kites flying in the sky and the trees moving to and fro when the wind blows.

How do kites fly in the sky? What causes the trees to move back and forth when the wind blows?

The movement of kites and the swaying of trees are caused by the pressure exerted on them by air.

What is this air pressure?

Air pressure refers to the force exerted per unit area of surface by the air that is directly above that surface.

When air blows, it exerts pressure on the face of a kite and pushes the kite up in the sky. Similarly, when the wind blows, it exerts pressure on a tree and makes it bend in the direction of the blowing air.

Let us perform an activity to understand this.

Please perform the activity under adult supervision.

• Pour water into a metal can and boil over a stove keeping the can uncovered.
• After boiling for 2-3 minutes, cover the can with a lid.
• Using tongs, carefully place the can in the sink.
• Pour cold water over it.
  What do you observe? When cold water is poured over the can, it causes the can to crush.
  Water vapour condenses when cooled. It has more pressure. Therefore, when cooled, the vapour condenses, reducing the pressure. This compresses the metal can. Therefore, air exerts pressure.
  Let us look at some more instances where we feel the air pressure.
  • Cycling or rowing against the wind is difficult because of the pressure exerted by air.
  • The tyres of vehicles are filled with air, thereby exerting pressure. This makes it possible for them to hold and carry weight.
    
    Let us now explore another important property of air by performing an activity.
    Place a small paper ball inside the neck of a transparent bottle as shown and forcefully blow air into it.What do you observe?
    It is observed that it is difficult to push the small paper ball inside the bottle by blowing.
    This is because the speed of the wind is higher at the mouth of the bottle, thereby resulting in reduced air pressure near the mouth of the bottle.
    On the contrary, the wind inside the bottle has a low speed and therefore, a higher pressure as compared to that at the mouth of the bottle. This higher air pressure inside the bottle prevents the paper ball from entering the bottle.
    Therefore, high wind speed has low pressure and low wind speed has high pressure.
    Let us perform a small experiment.
    
    
    Let us look at another demonstration of this property. Place a strip of paper between your lips and forcefully blow out air. You will observe that the paper strip moves upwards.