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⚛️ Physics  ·  Mechanical Properties of Fluids  ·  NEET & JEE

A barometer uses mercury (ρ = 13600 kg/m³). Atmospheric pressure 10⁵ Pa corresponds to a mercury column height of:

Answer: 0.735 m (73.5 cm).

  • A 0.735 m (73.5 cm)
  • B 10 m according to standard textbooks
  • C 13.6 m in general practice
  • D 1 m as frequently described

Correct answer: A. 0.735 m (73.5 cm)

Explanation: P = ρgh → h = P/(ρg) = 10⁵/(13600 × 10) = 10⁵/1.36×10⁵ ≈ 0.735 m = 73.5 cm ≈ 760 mmHg.

Continuity Equation: Narrow Pipe → Faster FlowA₁, v₁ (wide, slow)A₂, v₂ (narrow, fast)A₁v₁ = A₂v₂ (continuity)Smaller area → higher speed → (by Bernoulli) LOWER pressure

Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.

Concept context

Pressure, buoyancy, Bernoulli equation, viscosity, and surface tension.

Read the full Mechanical Properties of Fluids notes →