Mechanical Properties of Fluids

Key Terms:

• Density: Mass per unit volume. Remember this as the amount of matter packed into a given space

• Specific gravity: Ratio of the density of a fluid to the density of water at 4°C. Think of it as how heavy a fluid is compared to water

• Compressibility: Measure of the ability of a fluid to change volume under pressure. Imagine squeezing a balloon - the more compressible, the easier it is to squeeze.

• Bulk modulus: Measure of the resistance of a fluid to uniform compression. Think of it as the fluid’s stiffness - the higher the bulk modulus, the harder it is to compress.

• Viscosity: Measure of the resistance of a fluid to flow. Remember this as the “thickness” or “gooeyness” of a fluid

• Newtonian fluid: A fluid whose viscosity is constant. Think of water or honey - their viscosity doesn’t change much with the rate of flow

• Non-Newtonian fluid: A fluid whose viscosity varies with the rate of shear stress. Imagine ketchup or toothpaste - they are thicker when you apply a slow force, but thinner when you apply a quick force.

• Laminar flow: Fluid flow in which layers of fluid move smoothly past each other without mixing. Think of water flowing through a pipe - the layers of water move smoothly without mixing.

• Turbulent flow: Fluid flow in which layers of fluid mix together. Imagine water flowing through a rapids - the layers of water mix and create turbulence

• Bernoulli’s equation: Equation relating the pressure, velocity, and height of a fluid in steady flow. Remember this as the “energy equation” for fluids

• Pascal’s law: Pressure applied to a confined fluid is transmitted equally to every point in the fluid. Picture a hydraulic system - the pressure applied to one part of the fluid is transmitted to all other parts

• Archimedes’ principle: Buoyant force acting on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. Think of a boat floating on water - the buoyant force keeps it afloat.

• Capillarity: Rise or fall of a liquid in a narrow tube due to surface tension. Visualize a straw in a glass of water - the water rises inside the straw due to capillarity.

• Surface tension: Force acting at the surface of a liquid that tends to contract the surface and minimize its area. Imagine a water droplet - it takes the shape of a sphere to minimize surface tension

• Cohesion: Force of attraction between molecules of the same substance. Think of water droplets holding together due to cohesion

• Adhesion: Force of attraction between molecules of different substances. Visualize water sticking to glass due to adhesion

• Wetting: Spreading of a liquid on a surface due to adhesion. Imagine water spreading on a clean glass surface due to wetting.

• Non-wetting: Non-spreading of a liquid on a surface due to cohesion. Think of water beading up on a waxed car surface due to non-wetting.