Describe how pressure varies with force and area in the context of everyday examples

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Pressure

1.8 Pressure

Learning Objective

Describe how pressure varies with force and area in the context of everyday examples.

Definition of Pressure

Pressure is the amount of force applied per unit area on a surface.

Mathematically, \$p = \frac{F}{A}\$ where \$p\$ is pressure (Pa), \$F\$ is the force (N) and \$A\$ is the area (m²).

Units and Symbols

  • SI unit: pascal (Pa) = N·m⁻²

  • Other common units: kilopascal (kPa), bar (1 bar = 100 kPa), atmosphere (1 atm ≈ 101.3 kPa)

Relationship Between Force, Area and Pressure

From the formula \$p = \frac{F}{A}\$ we see:

  1. If the force \$F\$ increases while the area \$A\$ stays the same, the pressure \$p\$ increases proportionally.

  2. If the area \$A\$ increases while the force \$F\$ stays the same, the pressure \$p\$ decreases.

  3. For a given pressure, a larger force can be produced by increasing the area, and a smaller force can be produced by decreasing the area.

Everyday Examples

ExampleForce (N)Contact Area (m²)Resulting Pressure (Pa)Explanation
High‑heeled shoe≈ 500≈ 0.0005≈ 1.0 × 10⁶Small area → high pressure → can sink into soft ground.
Snowshoe≈ 500≈ 0.25≈ 2.0 × 10³Large area spreads weight → low pressure → prevents sinking.
Sharp knife cutting bread≈ 20≈ 0.0001≈ 2.0 × 10⁵Very small edge area gives high pressure, breaking the bread’s structure.
Hydraulic car jack (small piston)1000.0011.0 × 10⁵Same pressure transmitted to larger piston (area 0.01 m²) gives output force 1000 N.

Application: Hydraulic Press

In a hydraulic system the pressure is the same throughout the fluid. If a small force \$F1\$ is applied to a piston of area \$A1\$, the pressure is \$p = \frac{F1}{A1}\$. This pressure acts on a second piston of area \$A2\$, producing an output force \$F2 = pA2 = \frac{F1}{A1}A2\$.

Thus, by using a large \$A2\$ relative to \$A1\$, a small input force can lift a much heavier load.

Key Points to Remember

  • Pressure increases with greater force and decreases with larger area.

  • Everyday objects exploit this relationship: sharp tools use small areas to achieve high pressure; snowshoes use large areas to keep pressure low.

  • Hydraulic systems rely on the principle that pressure is transmitted equally in all directions.

Suggested diagram: Illustration showing (a) a high‑heeled shoe sinking into sand, (b) a snowshoe distributing weight, and (c) a hydraulic press with two pistons of different areas.