Page 74 - Innovator's Science-8

It makes difficult for us to drive while moving against the wind .

2. Similarly, while rowing a sail boat, we find that it is easier to row the
sail boat in the direction of wind. It is difficult to row a sail boat
opposite side to the direction of wind.

3. When we are flying a kite, then the wind coming from our back side
helps the kite to fly higher and higher. It is the blowing wind which
exerts the pressure on the kite to fly higher.

4. When air is filled in a bicycle tube with the help of an air pump, it
inflates. Here it is the pressure exerted by air filled in a bicycle tube

which keeps the tube ‘tight’ and makes the bicycle tube feel hard.
However, if we fill too much air, the bicycle tube bursts because it
cannot withstand huge pressure of air.

All the above experiences show that moving air (or wind) exerts a
force (or pressure) on objects.

Activity-1 Aim: To observe that warm air expands and becomes lighter.
Materials Required: An empty glass bo le, a balloon, a bowl of hot water, a bowl of
Experiential Learning
cold water.
Procedure:

1. Stretch the balloon and fit it ghtly over the mouth of the empty glass bo le.
Science 2. Place the bo le in a bowl of hot water and observe the balloon.
Bytes
3. A er a few minutes, take the bo le out and place it in cold water.
Testimony
When air inside the bo le 4. Observe what happens to the balloon.
is heated, the molecules Observa on: The balloon inflates when the bo le is in hot water and shrinks when
move faster and need placed in cold water.
more space. This is why Conclusion: When air is heated, it expands and becomes lighter. When cooled, it
the balloon inflates - it's
the same principle that contracts and becomes denser.
makes hot air balloons This explains why warm air rises an important cause of winds and air pressure
rise.
differences.

WHAT CAUSES AIR PRESSURE?
The Earth is completely surrounded by air. The gravity (pull of the Earth)

pulls the molecules of air and other atmospheric gases
downwards towards the ground. The upper layers of
the atmosphere rest on the lower layers, thus exerting

pressure on the surface of the Earth.
Air is compressible . The lowest layer is the most
compressed and is, therefore, the dense .

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Science-8