NCERT Solutions Curiosity Chapter 6 .2 Pressure Exerted by Air — Activity 6.4: Let us perform

Book page 876 Updated on2026-09-05

Q1.

Do you realise that it sticks to the surface?

Answer

Yes — once pressed firmly on a smooth flat surface, the sucker stays there on its own.

Pressing the cup down pushes almost all the air out from between the cup and the surface. Very little air is left inside, so the air pressure inside the sucker becomes much lower than the atmospheric pressure outside. The outside air now presses the sucker against the surface from every side, and it is held in place.

Why the surface must be smooth: on a rough surface, air keeps leaking back in through the tiny gaps between the cup and the bumps. The pressure inside never falls, no pressure difference is set up, and the sucker will not stick.

Q2.

Now, try to pull it off. Do you find it difficult to pull it off?

Answer

Yes, it is surprisingly hard to pull off — and how hard tells us how large the atmospheric pressure is.

To lift the sucker you must overcome the pressure difference between the air outside it and the thin air inside it. Your pull has to be greater than the force with which the atmosphere is holding the cup down. Since the atmosphere presses on every square centimetre with a large force, even a small sucker takes a firm tug.

Air outside: atmospheric pressure (high)
Air trapped inside: pressure reduced by pressing the cup down (low)
Sucker held on by the pressure difference
To remove it: applied force must overcome that difference

Did you know? The book puts a number to the atmosphere's push: over an area of just 15 cm × 15 cm, the air column presses with about 2250 N — the force of gravity on a 225 kg mass.

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