NCERT Solutions Curiosity Chapter 6 .2 Pressure Exerted by Air — Activity 6.3: Let us explore

Lifting is easier when the plate is covered with the folded chart paper. With the unfolded sheet, distinctly more effort is needed.

The two chart-paper sheets are identical — about 70 cm × 56 cm each — so their weights are the same. Folding one twice does not make it lighter; it only makes it cover a smaller area of the plate. Yet the folded sheet lifts easily and the spread-out sheet resists. The difference cannot come from weight, so it must come from area.

That air presses down on the covering sheet, and that the total force it exerts grows as the area of the sheet grows. Force per unit area is pressure — so air exerts pressure.

Set the two cases side by side. Same sheet, same weight, same stick, same plate. The only thing that changed was how much area the sheet presented to the air above it. When that area was made larger, the effort needed to lift the plate became larger. Something above the sheet must therefore be pushing down on it, and the push must be proportional to the area — which is precisely how a pressure behaves.