Physics (5054)
Topic 4 of 8Cambridge O Levels

Pressure

Why sharp heels sink in soft ground but flat shoes don't

Pressure is the force acting per unit area.


Formula: Pressure = Force / Area (P = F/A)

Unit: Pascal (Pa). 1 Pa = 1 N/m²


Why Pressure Matters:

  • A knife cuts because the blade has a tiny area, creating huge pressure
  • Snowshoes spread your weight over a large area, reducing pressure so you don't sink
  • Heavy trucks have many tyres to spread weight and reduce road pressure

    • Pressure in Liquids:

    Liquid pressure increases with depth. P = ρgh (density × gravity × height)

  • This is why dams are thicker at the bottom
  • Why your ears hurt when diving deep in a pool

    • Atmospheric Pressure:

    The atmosphere exerts pressure on us (about 101,325 Pa at sea level). We don't feel it because our bodies push back equally. Atmospheric pressure decreases with altitude — that's why air is thinner on mountains.


    Hydraulic Systems:

    Liquids are incompressible, so pressure applied to one part transmits equally to all parts. This is used in hydraulic brakes and jacks.

    Key Points to Remember

    • 1Pressure = Force / Area (P = F/A) in Pascals
    • 2Liquid pressure = density × g × depth (P = ρgh)
    • 3Atmospheric pressure is ~101,325 Pa at sea level
    • 4Hydraulic systems use incompressible liquids to transmit pressure

    Pakistan Example

    Rickshaw Tyres vs Truck Tyres

    A Suzuki Bolan (about 800 kg) has 4 narrow tyres, each with maybe 0.02 m² of contact area. Total area = 0.08 m². Pressure on road = (800×10)/0.08 = 100,000 Pa. A heavy truck (20,000 kg) with 18 tyres, each 0.05 m² contact = 0.9 m² total. Pressure = (20,000×10)/0.9 = 222,222 Pa. Even with 18 tyres, trucks still damage roads more! That's why overloaded trucks destroy Pakistani highways.

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    Energy & Work DoneThermal Physics