NCERT Solutions for Class 8th Science Chapter 6 .2 Pressure Exerted by Air — Activity 6.3: Let us explore
Book page 856 Updated on2026-09-19
Q1.
In which case is the lifting easier, with the folded or the unfolded chart paper covering the paper plate?Fig. 6.9 (b) and (c), page 86 — redrawn: (b) the inverted paper plate covered with the chart paper folded twice; (c) the same plate covered with the unfolded chart paper. In both, the stick passes out through a hole at the centre of the sheet.
Answer
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.
Q2.
What can you infer from this?
Answer
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.
Folded sheet → small area → small downward force → easy to lift Unfolded sheet → large area → large downward force → hard to lift Force ∝ Area, so Force / Area = a constant pressure — the atmospheric pressure
Why it happens: air is not empty space. It is a real substance surrounding the plate, and it presses on every object it touches. The pressure exerted by the air around us is called the atmospheric pressure.