How liquid pressure works
A liquid exerts pressure on any surface in contact with it because of its weight. This pressure acts in all directions and depends only on the depth, the density of the liquid and gravity — not on the shape or width of the container.
Key formula / 关键公式
Liquid pressure: P = ρgh
where ρ = density (kg m⁻³), g = gravitational acceleration (≈ 10 m s⁻²), h = depth below the surface (m). Pressure is measured in pascals (Pa).
General definition: P = F / A
Key features
- Pressure increases with depth — deeper points feel more pressure.
- Pressure increases with liquid density.
- Pressure does not depend on the surface area or shape of the container.
- At the same depth in the same liquid, the pressure is equal in all directions.
Worked example / 例题
Find the pressure at a depth of 5 m in water (ρ = 1000 kg m⁻³, g = 10 m s⁻²).
P = ρgh = 1000 × 10 × 5 = 50 000 Pa (50 kPa).
Worked example / 例题
A diver is 20 m deep in sea water (ρ = 1025 kg m⁻³, g = 10 m s⁻²). Find the water pressure on the diver.
P = ρgh = 1025 × 10 × 20 = 205 000 Pa (about 205 kPa).
Remember / 记住
- P = ρgh — depth and density matter, not container shape.
- Pressure acts equally in all directions at a given depth.
- Dam walls are built thicker at the bottom because pressure is greater there.