Pressure in Solids, Liquids and Gases
Pressure in Solids, Liquids and Gases
Pressure gives 1 to 2 questions and connects directly to everyday examples that a class 8 teacher is expected to explain. The formula is one line; the marks come from knowing how pressure behaves differently in solids, liquids and gases.
The Basic Relationship
Pressure = Force ÷ Area | SI unit: pascal (Pa) = N/m²
For the same force, a smaller area gives greater pressure. This one relationship explains most of the standard examples.
- A sharp knife cuts better because the small edge area raises the pressure.
- A camel's broad feet spread its weight, lowering the pressure on sand.
- School bags have wide straps so the same weight presses over a larger area.
- Nails and pins are pointed for exactly the same reason.
✗ A sharp knife cuts better because it applies more force | ✓ The force is the same; the small area raises the pressure
Pressure in Liquids
Pressure at depth h = h × d × g
- Liquid pressure increases with depth and with the density of the liquid.
- It does not depend on the shape of the container or on the total quantity of liquid.
- At the same depth, pressure acts equally in all directions.
- Dam walls are built thicker at the bottom because pressure is greatest there.
The independence from container shape is a favourite question, because it seems wrong to most students. Two vessels of different shapes filled to the same depth with the same liquid have the same pressure at the base.
Buoyancy and Archimedes' Principle
An object immersed in a fluid experiences an upward force called buoyancy or upthrust.
Upthrust = weight of the fluid displaced by the object
- An object floats if its density is less than that of the liquid, and sinks if it is greater.
- An iron nail sinks but an iron ship floats, because the ship's shape makes its average density lower than water.
- An object weighs less in water than in air, and the apparent loss of weight equals the upthrust.
- This is why swimming is easier in sea water than in a river — sea water is denser, so it gives more upthrust.
Atmospheric Pressure
The atmosphere exerts pressure in all directions because of the weight of the air above. It is measured with a barometer, and decreases as altitude increases.
- A rubber sucker sticks because atmospheric pressure holds it against the surface once the air is pushed out.
- Drinking through a straw works by lowering the pressure inside, letting atmospheric pressure push the liquid up.
- Nose bleeding at high altitude happens because outside pressure falls while blood pressure does not.
- Cooking takes longer on mountains because the lower pressure lowers the boiling point of water.
Each of these can appear as a one-line question asking for the reason, so learn the explanation rather than just the example.
द्रव का दाब गहराई पर निर्भर करता है, बर्तन के आकार पर नहीं।
TRE pointer: The examiner's preferred format here is a why question about an everyday object, which is exactly what a teaching post demands. The most repeated single fact is that liquid pressure depends on depth and density but not on the shape of the vessel. The floating iron ship is the second. Because these are reasoning items, the wrong options are all plausible-sounding explanations, so a half-remembered answer is expensive when a wrong one costs −1/3.
60-Second Recap
- Pressure = force ÷ area; smaller area means greater pressure.
- Liquid pressure = h × d × g; it grows with depth and density.
- Liquid pressure does not depend on the shape of the container.
- Upthrust equals the weight of the fluid displaced.
- An object floats when its density is less than the liquid's; a ship floats by shape.
- Atmospheric pressure falls with altitude and is measured by a barometer.
Frequently Asked Questions
Does liquid pressure depend on the shape of the container?
No. It depends only on the depth, the density of the liquid and gravity. Two containers of very different shapes, filled with the same liquid to the same depth, have exactly the same pressure at the base. This result surprises students and is asked often.
Why does an iron ship float when an iron nail sinks?
Because the ship is hollow, so the water it displaces weighs more than the ship itself, and its average density is lower than that of water. The nail is solid, its density is greater than water's, and the upthrust cannot support its weight.
Why is cooking slower at high altitudes?
Because atmospheric pressure falls with altitude, and lower pressure lowers the boiling point of water. Water then boils at a temperature below a hundred degrees, so food receives less heat and takes longer to cook. Pressure cookers solve this by raising the internal pressure.
Why do camels have broad feet?
To spread their weight over a larger area, which reduces the pressure on the sand and stops them sinking. It is the same principle as a wide school-bag strap or a tractor's broad tyres. The force stays the same; only the area, and therefore the pressure, changes.