MECHANICAL PROPERTIES OF FLUIDS
Frequently Asked Questions
Mechanical properties of fluids describe how liquids and gases respond to forces, including their ability to flow, transmit pressure, resist motion, and show surface effects like viscosity and surface tension.
A fluid is a substance that cannot resist shear stress in static condition and continuously deforms under even a small tangential force. Liquids and gases are fluids.
Pressure is the normal force exerted per unit area by a fluid on any surface in contact with it, given by P = F/A.
Pressure increases with depth according to P = P0 + ?gh, where ? is density, g is acceleration due to gravity, and h is depth.
Pascal’s law states that pressure applied to an enclosed fluid is transmitted equally and undiminished to all parts of the fluid and the container walls.
Pascal’s law is applied in hydraulic lifts, hydraulic brakes, hydraulic presses, and hydraulic jacks.
Buoyant force is the upward force exerted by a fluid on an immersed body, opposing the weight of the body.
A body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by it.
Objects float if the buoyant force equals or exceeds their weight, usually when their average density is less than the fluid density.
Viscosity is the internal resistance of a fluid to the relative motion between its layers.
The coefficient of viscosity is the force per unit area required to maintain a unit velocity gradient between two parallel layers of a fluid.
Stokes’ law states that the viscous force on a small sphere moving in a fluid is F = 6p?rv.
Terminal velocity is the constant maximum velocity attained by a body falling through a viscous fluid when net force becomes zero.
Terminal velocity is given by v = (2r²(? - s)g)/(9?).
Surface tension is the property of a liquid surface that makes it behave like a stretched elastic membrane.
Surface energy is the work required to increase the surface area of a liquid by unit area.
Surface tension arises due to unbalanced intermolecular forces on surface molecules.
Capillarity is the rise or fall of a liquid in a narrow tube due to surface tension and adhesive forces.
h = 2Tcos? / (?gr), where T is surface tension and r is tube radius.
The sum of pressure energy, kinetic energy, and potential energy per unit volume remains constant for steady fluid flow.
It is used in airplane wings, venturimeters, atomizers, carburetors, and blood flow measurement.
Streamline flow is smooth fluid motion in which velocity at any point remains constant with time.
Turbulent flow is irregular fluid motion characterized by eddies and random velocity changes.
Reynolds number is a dimensionless quantity used to predict the nature of fluid flow.
Viscosity affects lubrication, blood circulation, fuel flow, and industrial fluid transport.
Viscometers, lubrication systems, shock absorbers, and dampers rely on viscosity.
Excess pressure is the additional pressure inside a bubble due to surface tension.
Excess pressure = 4T / r.
Excess pressure = 2T / r.
Numericals on pressure variation, buoyancy, terminal velocity, viscosity, capillarity, and Bernoulli’s equation are frequently asked.
Gauge pressure is the pressure measured relative to atmospheric pressure and is given by Pg = P - Patm.
Absolute pressure is the total pressure measured relative to vacuum and equals atmospheric pressure plus gauge pressure.
In a fluid at rest, intermolecular collisions transmit force equally in all directions, resulting in pressure acting perpendicular to surfaces.
A hydraulic lift is a device based on Pascal’s law that multiplies force using pressure transmission through an incompressible fluid.
Pressure increases with depth, so dams are made thicker at the bottom to withstand greater water pressure.
Upthrust depends on the density of the fluid, acceleration due to gravity, and the volume of fluid displaced.
Ice floats because its density is less than that of water, so buoyant force balances its weight before full immersion.
Relative density is the ratio of the density of a substance to the density of water at 4°C.
In liquids, viscosity decreases with increase in temperature.
In gases, viscosity increases with increase in temperature.
Laminar flow is fluid motion in which adjacent layers slide smoothly over one another without mixing.
Flow generally becomes turbulent when Reynolds number exceeds about 2000.
A streamline is a path traced by a fluid particle in steady flow such that the tangent at any point gives the velocity direction.
The equation of continuity states that Av = constant for incompressible fluid flow, where A is area and v is velocity.
A venturimeter uses Bernoulli’s principle to measure flow speed and discharge.
According to Bernoulli’s principle, increase in fluid speed results in decrease in pressure.
High-speed wind above the roof reduces pressure, causing higher pressure inside to lift the roof upward.
The SI unit of coefficient of viscosity is pascal second (Pa·s).
The SI unit of surface tension is newton per metre (N/m).
Small insects walk on water due to surface tension providing sufficient upward force.
Angle of contact is the angle between the tangent to the liquid surface and the solid surface at the point of contact.
Mercury has greater cohesive force than adhesive force with glass, causing capillary depression.
Water rises due to strong adhesive forces between water and glass and surface tension.
Excess pressure inside a liquid drop is given by 2T/r due to surface tension.
The fluid is incompressible, non-viscous, and flowing steadily along streamlines.
Real fluids possess viscosity and energy losses, which violate ideal assumptions.
Viscous drag is the resistive force experienced by a body moving through a viscous fluid.
Terminal velocity helps determine viscosity of fluids and explains motion of particles in fluids.
Board exams focus on derivations, conceptual explanations, and simple numericals.
Competitive exams emphasize numericals, conceptual traps, and application-based problems.
