Winds, Storm and Cyclones
Science Class Seven
Activity Solution 8.1 to 8.3
Activity: 8.1
• Take a tin can with a lid.
• Fill it approximately half with water.
• Heat the can on a candle flame till the water boils.
• Let the water boil for a few minutes.
• Blow out the candle.
• Immediately put the lid tightly on the can.
• Be careful in handling the hot can.
• Put the can carefully in a shallow metallic vessel or a washbasin.
• Pour fresh water over the can.
Question: What happens to the shape of the can?
Solution
The shape of the can gets distorted inwards.
Question: Can you guess why the shape of the can gets distorted?
Solution
The shape of the can gets distorted because
(a) Air inside the can gets expelled out while boiling the water along with the vapour.
(b) And after cooling, i.e. after pouring the water over the hot can, the vapour inside the can gets condensed and changed into water. This reduces air pressure inside the can, while the pressure of the air outside the can remains the same. Since air exerts pressure from outside, so, the can gets distorted.
Recall now some of your experiences.
When you fly a kite, does the wind coming from your back help?
Solution
Yes, while flying a kite, the wind coming from the back side helps in flying the kite.
This happens because the wind coming from back side exerts pressure on the kite and it helps the flying of the kite.
Question: If you are in a boat, is it easier to row it if there is the wind coming from behind you?
Solution
Yes, the wind coming from behind helps in rowing a boat.
This happens because the wind coming from behind the boat exerts pressure on it which eases the rowing of the boat forwards.
Question: Do you find it difficult to ride a bicycle against the direction of the wind?
Solution
Yes, it is difficult to ride a bicycle against the direction of the wind.
This happens because air (wind) exerts pressure.
The wind coming from the front side of the cycle exerts pressure, which makes it difficult to ride a bicycle against the direction of the wind.
Question: You know that we have to fill air into the bicycle tube to keep it tight. Also, you know that a bicycle tube overfilled with air may burst. What is the air doing inside the tube?
Solution
Air exerts pressure. Because the pressure of the air inside the bicycle tube keeps it inflated. This makes the ride of a cycle easy.
This is the cause that there are tubes in the wheel of all types of vehicles.
Activity 8.2
• Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle.
• Hold the empty bottle on its side and place the paper ball just inside its mouth.
• Now try to blow on the ball to force it into the bottle.
• Try the activity with bottles of different sizes.
• Challenge your friends if they can force the paper ball in by blowing into the bottle.
Paheli and Boojho are thinking about the following question:
Question: Why is it difficult to force the paper ball into the bottle?
Solution
High speed wind is always accompanied by low pressure. This means high speed wind always creates low pressure zone also.
When air is blown in the bottle after keeping a crumbled small piece of the paper ball of the size smaller than the mouth of an empty bottle near the mouth of the bottle, a low pressure zone is created near the mouth of the bottle compare to the air pressure inside the bottle.
Because the air pressure inside the bottle is greater than that near the mouth, so the paper ball does not go inside the bottle, rather it is expelled out.
This happens because of the pressure difference between the inside and near the mouth of the bottle.
Activity: 8.3
Take two balloons of approximately equal size.
Put a little water into the balloons. Blow up both the balloons and tie each one to a string.
Hang the balloons 8–10 cm apart on a cycle spoke or a stick.
Blow in the space between the balloons.
Question: What did you expect?
Solution
We expect that balloons will go apart from each other while blowing the air in the space between the balloons.
Question: What happens?
Solution
When air is blown in the space between the balloons, the balloons come closer to each other.
This happens because high speed wind creates a low pressure zone.
When air is blown in the space between the balloons, because of high speed wind a zone of low pressure is created in the space between the balloons compare to the air pressure on both sides. The higher air pressure from both sides of the balloons pushes balloons inside, and because of this balloons come closer to each other.
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