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NCERT Solutions Curiosity Chapter 6 .3 Formation of Wind — Activity 6.5: Let us observe

Book page 886 Updated on2026-09-05

Q1.
Predict what would happen to the balloons.
Answer

Prediction: the inflated balloon will shrink and the uninflated one will swell, until both are of about the same size.

The reason for predicting this is the rule we already have: air moves from a region of high pressure to a region of low pressure. The inflated balloon holds air at a pressure higher than that in the limp balloon at the other end of the straw. As soon as the mouth is released, the straw becomes a path between them, so air should travel through it from the inflated balloon to the uninflated one.

Tip: a prediction is worth writing down before the activity. If the observation matches it, you have confirmed the rule; if it does not, you have found something new to explain.
Q2.
Observe what happens to both the balloons. Did it happen as predicted?
Answer

Yes, it happens as predicted. The inflated balloon starts getting smaller and the uninflated one starts filling up.

Air travels along the straw from the inflated balloon into the limp one. You can often hear the rush of air and see both balloons changing at the same moment. After a short while the movement slows and stops, and the two balloons are left almost equal in size.

inflated high pressure straw air flows this way uninflated low pressure
Air travels through the straw from the high-pressure balloon to the low-pressure one — the same rule that makes wind blow.
Q3.
Do you observe any change in the size of the balloons? Write down your observations.
Answer

Yes. Both balloons change size, and they change in opposite ways.

  • The inflated balloon shrinks — it becomes visibly smaller as air leaves it.
  • The uninflated balloon swells — it fills out as air arrives in it.
  • After some time both balloons are almost the same size, and no further change is seen.

Note also that the balloons do not become equal instantly. The change is fastest at the beginning, when the difference between the two pressures is largest, and it slows down as the pressures come closer together.

Q4.
What can be the reason for the change in the sizes of the balloons?
Answer

The air pressure in the inflated balloon is higher than the air pressure in the uninflated balloon, so air moves from the inflated balloon into the uninflated one through the straw.

The balloon that loses air shrinks; the balloon that gains air expands. Nothing is being blown by mouth at this stage and no rubber is being stretched by hand — the only cause available is the difference in pressure between the two ends of the straw.

Q5.
Do you notice that after some time both the balloons attain almost the same size and the flow of air stops? Why does the air flow stop?
Answer

The flow stops because the pressures in the two balloons have become equal. With no pressure difference left, there is nothing to drive the air along the straw.

Follow the process through. At the start the pressure difference is large and air moves quickly. As air leaves the inflated balloon its pressure falls; as air enters the other balloon its pressure rises. The gap between the two narrows, the flow slows, and when the two pressures match, the movement ceases altogether. At that stage both balloons are of almost the same size.

The conclusion of the activity: air moves from a region of high air pressure to a region of low air pressure, and it keeps moving only as long as a pressure difference exists. This is exactly why wind blows — and why it dies down when the pressure evens out.
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