Mechanical Properties of Fluids – Core Vocabulary

These vocabulary flashcards cover fundamental terms and principles from the lecture chapter on Mechanical Properties of Fluids, including pressure, fluid dynamics, viscosity, surface tension, and related applications. Review them to reinforce conceptual understanding and exam readiness.

Fluid

A substance (liquid or gas) that can flow, offering very little resistance to shear stress.

Pressure (P)

Normal force exerted per unit area; P = F⁄A (SI unit = pascal, Pa).

Pascal (Pa)

The SI unit of pressure; 1 Pa = 1 N m⁻².

Atmosphere (atm)

A common pressure unit equal to 1.013 × 10⁵ Pa (standard atmospheric pressure at sea level).

Torr

Pressure unit equal to 1 mm Hg; 1 torr ≈ 133 Pa.

Bar

Pressure unit; 1 bar = 10⁵ Pa.

Gauge Pressure

Pressure measured relative to atmospheric pressure (P – Pₐ).

Absolute Pressure

Total pressure measured relative to vacuum; Pₐ + gauge pressure.

Density (ρ)

Mass per unit volume of a substance; ρ = m⁄V (SI unit = kg m⁻³).

Relative Density (Specific Gravity)

Ratio of the density of a substance to the density of water at 4 °C; dimensionless.

Pascal’s Law

Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and container walls.

Hydrostatic Pressure

Pressure at depth h in a fluid; P = Pₐ + ρgh.

Hydrostatic Paradox

Pressure at a given depth depends only on depth, not on container shape or area.

Barometer

Instrument (often mercury‐filled) used to measure atmospheric pressure.

Manometer

U-tube device that measures pressure difference by liquid column height.

Hydraulic Machine

Device (e.g., lift, brake) that transmits and multiplies force via an incompressible fluid using Pascal’s law.

Hydraulic Lift

Two-piston system where small input force on a small area produces large output force on a larger area.

Hydraulic Brakes

Automobile braking system using fluid pressure to apply equal braking force to all wheels.

Streamline Flow (Steady or Laminar Flow)

Fluid motion where velocity at each point remains constant in time; streamlines do not cross.

Turbulent Flow

Irregular, chaotic fluid motion occurring above a critical speed.

Continuity Equation

For incompressible flow, A v = constant; expresses conservation of mass in a streamline tube.

Bernoulli’s Principle

In steady, non-viscous, incompressible flow, P + ½ρv² + ρgh = constant along a streamline.

Bernoulli’s Equation

Mathematical statement of Bernoulli’s principle relating pressure, kinetic, and potential energy per unit volume.

Torricelli’s Law (Speed of Efflux)

Speed of liquid exiting a hole equals √(2gh), analogous to free-fall speed.

Dynamic Lift

Upward force on a body moving through a fluid due to pressure differences (e.g., airplane wing).

Magnus Effect

Dynamic lift produced on a spinning body moving through a fluid, causing curved trajectories.

Viscosity (η)

Internal friction of a fluid; ratio of shear stress to rate of shear strain (SI unit = Pa s or poiseuille).

Coefficient of Viscosity

Proportionality constant η in shear stress = η (velocity gradient).

Poiseuille (Pl)

SI derived unit of viscosity equal to 1 Pa s.

Stokes’ Law

Viscous drag on a sphere: F = 6π η a v (a = radius, v = speed).

Terminal Velocity

Constant speed reached by a falling object when gravity is balanced by viscous drag and buoyancy.

Ideal Fluid

Hypothetical fluid that is incompressible, non-viscous, and experiences steady, irrotational flow.

Surface Tension (S)

Force per unit length (or energy per unit area) acting along a liquid surface; tends to minimize surface area.

Surface Energy

Extra potential energy possessed by molecules at a liquid surface compared to interior molecules.

Angle of Contact (θ)

Angle between tangent to liquid surface and solid surface inside the liquid; determines wetting behaviour.

Capillary Rise (or Fall)

Elevation or depression of a liquid in a narrow tube due to surface tension; h = 2S cos θ⁄(ρ g a).

Drop Pressure (Laplace Pressure)

Excess pressure inside a spherical drop: ΔP = 2S⁄r (or 4S⁄r for a bubble with two surfaces).

Hydrostatic Lift

Buoyant force experienced by an object submerged in a fluid (Archimedes’ principle).

Critical Speed

Maximum speed below which fluid flow remains laminar; above it flow becomes turbulent.

Laminar Layer

Thin fluid layer adjacent to a surface where viscous effects dominate and flow is orderly.

Compressibility

Measure of fractional volume change under pressure; gases have high, liquids low compressibility.

Poise

CGS unit of viscosity; 1 poise = 0.1 Pa s.

Hydrostatic Equilibrium

Condition where pressure gradients balance body forces (e.g., gravity) in a fluid at rest.

Efflux

Outflow of a fluid from an orifice or opening.

Aerofoil

Shaped surface (aircraft wing) designed to produce lift by creating pressure differences in airflow.