CBSE Class 8 – Force and Pressure (PRESSURE Notes)
1. Pressure – Concept
Definition: Pressure is defined as the force acting on a unit area.
Important points:
- For a given force, smaller area → higher pressure.
Example: Sharp blade cuts better than blunt one; girl wearing pencil heels exerts greater pressure than an elephant.
- For a given force, larger area → lesser pressure.
Example: Lying on mattress spreads body weight → sinks less.
- For fixed area, greater force → greater pressure.
2. Examples of Pressure in Daily Life
- Skis are long and wide → large area, less pressure → skier does not sink into snow.
- Sledges without wheels → broad surface prevents sinking in snow.
- Tractor's rear tyres are broad → reduce pressure, prevent sinking into soil.
- Foundations of tall buildings are wide → prevent deep sinking.
- Camel's broad feet → exert less pressure → easy walking on desert sand.
- Knives, blades, axes are sharp at edges → small area, large pressure → easy cutting.
- Drawing pins, thumb pins → broad top for pressing, sharp edge for high pressure penetration.
3. SI Unit of Pressure
- SI unit: Newton per square metre (N/m²).
- This unit is also called Pascal (Pa) (named after scientist Blaise Pascal).
1 Pa = 1 N/m²
- Another common unit: Atmospheric pressure (atm).
1 atm = 76 cm = 760 mm of mercury column
4. Pressure Exerted by Liquids
Liquids exert pressure on the walls and bottom of the container.
Properties:
- Pressure increases with depth → demonstrated by water rushing out of holes at different heights.
Example: Dams are thicker at the base than at the top.
- Pressure is the same in all directions at the same depth.
Example: Water streams out equally in all directions from holes at same level.
- Pressure does not depend on the shape or size of container.
Example: Different shaped cans with same height → water streams reach equal distances.
- Pascal's Law: Pressure applied at any point on an enclosed liquid is transmitted equally in all directions.
Applications: Hydraulic press, hydraulic brakes, hydraulic jack, garage lifts.
5. Pressure Exerted by Gases (AIR Pressure)
- Air also exerts pressure because it has weight.
- The force exerted by air on 1 unit area of any object is called atmospheric pressure.
Examples / Activities
Funnel + rubber sheet → air sucked from inside, sheet depressed due to atmospheric pressure.
Tin can experiment → hot steam pushes out air, when cooled → inward atmospheric pressure crushes the can.
Drinking straw → sucking creates low pressure inside straw, atmospheric pressure pushes liquid up.
6. Measurement of Atmospheric Pressure
- Simple (Mercury) Barometer (by Torricelli):
- 1 m long glass tube filled with mercury, inverted in mercury trough.
- Mercury column falls and stops at 76 cm → balanced by atmospheric pressure.
- Space above mercury = vacuum (no air).
- At sea level: atmospheric pressure = pressure due to 76 cm column of mercury = 1 atm.
- Fortin's barometer → improved mercury barometer.
- Aneroid barometer → direct reading device, no liquid used.
Variation with depth/altitude
- Atmospheric pressure decreases with altitude (on going higher).
- Pressure by liquids increases with depth.
7. Physiological Effects of Atmospheric Pressure
- At ground level: palm (100 cm²) supports force ~100 kg weight due to air pressure, but we don't feel it because pressure inside and outside body balances.
- At high altitude: lower atmospheric pressure → sometimes causes nose bleeding due to rupture of blood vessels.
SUMMARY – "So Now You Know"
✅ Pressure = Force per unit area.
✅ SI unit = Newton per square metre = Pascal (Pa).
✅ Liquids exert pressure → increases with depth, same in all directions, independent of container shape.
✅ Pascal's Law → liquid transmits pressure equally in all directions.
✅ Air exerts pressure = Atmospheric Pressure. Measured using barometer.
✅ 1 atm = 76 cm = 760 mm mercury column.
✅ Atmospheric pressure decreases with altitude; liquid pressure increases with depth.
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