It is an important area of physics that explores how liquids and gases behave, both when they're still and when they're moving. It includes key ideas like pressure, buoyancy, viscosity, surface tension, and laws such as Pascal’s and Bernoulli’s. It also looks at different types of flow, like smooth (laminar) and chaotic (turbulent). These concepts help us understand everything from how airplanes fly and pumps work to how water flows in rivers and blood moves through our bodies.
1.0Definition of Fluid
Fluid Mechanics studies the behavior of fluids—liquids and gases—at rest and in motion. A fluid is a substance that continuously deforms under even a small shear stress and cannot resist shear force when at rest.
2.0Density of Liquid
Density (ρ)of any substance is defined as the mass per unit volume
Density=VolumeMass⇒ρ=vm
Relative Density
Relative Density (RD), also known as Specific Gravity, is the ratio of the density of a substance to the density of water at 4°C.
It is a dimensionless quantity, meaning it has no units, since it's a pure ratio.
In the CGS system, the density of water at 4°C is 1g/cm³, so the numerical value of RD equals the density in g/cm³.
In the SI system, the density of water at 4°C is 1000 kg/m³.
RD helps compare how heavy a substance is relative to water, without needing unit conversion.
3.0Pressure in a Fluid
Pressure in a Fluid is the force exerted perpendicular to any surface in contact with a fluid at rest (liquid or gas).
Although the fluid is at rest, its molecules are in constant motion, colliding with surfaces and creating pressure.
At any point inside the fluid, the forces on an imaginary surface are equal and opposite to maintain equilibrium.
Pressure P is defined as the normal force dF丄 per unit area dA: P=dAdF⊥
If pressure is uniform over a finite surface area A, then, P=dAdF⊥
The SI unit of pressure is the Pascal (Pa), where:1 Pa=1N/m2
Another common unit in meteorology is the Bar, with:1 Bar=105 Pa
Note: Fluid pressure acts perpendicular to any surface regardless of its orientation. Since pressure has no specific direction, it is a scalar quantity, unlike force, which is a vector with direction.
Atmospheric Pressure (P0): The pressure exerted by Earth’s atmosphere, varying with weather and altitude. Standard atmospheric pressure at sea level is 1.013 × 10⁵ Pa.
Absolute pressure and Gauge Pressure: Pressure above atmospheric pressure is called gauge pressure, while total pressure is absolute pressure.
Note: Absolute pressure is always greater than or equal to zero. While gauge pressure can be negative also.
4.0Variation of Pressure
Variation of Pressure at two points in horizontal plane
PA=PB
The pressure is the same at two points in the same horizontal level.
Variation of pressure at two points in different height depth
If two points in a fluid differ in depth by h , the pressure difference between them is given by:
PA=PB+ρgh
Variation of pressure at two points when fluid is in horizontal acceleration
PA−PB=lρa
lhA−hB=ga
tanθ=ga
In a horizontally accelerating fluid, the free surface tilts at an angle θ such that Tanθ=ga, and pressure varies along the direction of acceleration.
Variation of pressure at two points when fluid is in vertical acceleration
In a fluid accelerating vertically upward, the pressure difference between two points separated by height h is:
(PA−PB)=ρ(g+a)h
Special cases :
(1) If a is (–) ve i.e. the vessel is accelerating downward then, (PA−PB)=ρ(g−a)h
(2) If a is greater than g then fluid occupies the upper part of the container.
Variation of pressure when fluid is under both horizontal and vertical acceleration
ax=acosϕ,ay=asinϕ
Horizontal Pressure Difference (between two points A and B, separated by distance l ) ΔP=lρax=lρacosϕ
Vertical Pressure Difference (between two points separated by height h )ΔP=ρ(g+ay)h=ρ(g+asinϕ)h
Free Surface Inclination: The angle θ between the fluid's free surface and the horizontal is given bytanθ=g+asinϕacosϕ
Variation of pressure in a rotating fluid
When a liquid rotates with angular velocity ω, the free surface forms a paraboloid of revolution described by y=2gω2x2
The slope of the surface is, dxdy=tanθ=gω2x
The maximum height at the container’s edge hmax relates to minimum height at the center hmin as: hmax=hmin+2gω2R2
Volume conservation leads to: h0=hmax−4gω2R2
Thus, the rise in liquid level at the periphery equals the fall at the center.
Pressure variation at any point inside the liquid.
PB=P0+2ρω2x2
5.0Pascal's Law
Pressure applied to a confined fluid is transmitted equally throughout the fluid and to the walls of its container.
Hydraulic Lift
A small piston with area A1 applies force F1 on a confined fluid, creating pressure: P=A1F1
This pressure is transmitted equally to a larger piston with area A2
Since pressure is the same in both pistons, A1F1=A2F2⇒F2=F1A1A2
Because A2>A1, the force F2 on the larger piston is greater than F1
Hydraulic lifts multiply force by the ratio of piston areas.
Applications include dentist chairs, car lifts, jacks, elevators, and hydraulic brakes.
Important points in Pressure
At the same point in a fluid, pressure is the same in all directions. In the figure,
P1=P2=P3=P4
Forces acting on a fluid in equilibrium have to be perpendicular to its surface. Because it cannot sustain the shear stress.
In the same liquid pressure will be the same at all points at the same level. For example, in the figure:
ρ1h1=ρ2h2
h∝ρ1
Torricelli Experiment (Barometer) :It is a device used to measure atmospheric pressure. In principle any liquid can be used to fill the barometer, but mercury is the substance of choice because its great density makes possible an instrument of reasonable size.
P0=ρgh=(13.6×103)(9.8)(0.760)=1.01×105N/m2
Mercury barometer reads the atmospheric pressure P0 directly from the height of the mercury column.
Manometer :It is a device used to measure the pressure of a gas inside a container. The U-shaped tube often contains mercury.
P1=P2
P1 = pressure of the gas in the container (P)
P1 = atmospheric pressure (P0)+ρgh
P0=P0+ρgh
P−P0=ρgh⇒Gauge Pressure
ρ is the density of the liquid used in the U-tube.
6.0Archimedes Principle
When an object is immersed in a fluid, it appears to weigh less due to an upward buoyant force.
This force equals the weight of the fluid displaced by the object.
This phenomenon is known as Archimedes’ Principle.
Magnitude of Buoyant Force (F)=viρLg
vi= immersed volume of solid, L=density of liquid, g=acceleration due to gravity
7.0Law of Floatation
An object of volume V and density s floats in a liquid of density L with immersed volume Vi
At equilibrium, Weight=Buoyant Force⇒Vρsg=ViρLg⇒VVi=ρLρs
Percentage of Volume Immersed: VVi×100=ρLρs×100
Three Cases:
ρs<ρL The object floats partially; only a fraction is submerged.
ρs=ρL The object is fully submerged but floats at any depth.
ρs>ρL The object is denser than the liquid and sinks.
Apparent Weight in a Liquid
When a body is fully immersed in a liquid, it experiences loss in weight due to upthrust (buoyant force)
Relative Density (R.D) of the body: R.D=LossinweightinwaterWeightinair=ρwρs
Buoyant Force in an Accelerating Fluid:
When a body is immersed in a fluid inside an accelerating lift, buoyant force changes based on effective gravity: F=VρLgeff, where geff=g−a
Cases:
Lift accelerating upward: geff=g+a⇒Buoyant force increases
Lift accelerating downward geff=g−a⇒Buoyant force decreases
Free fall (a=g):geff=0⇒No buoyant force→ Objects appear weightless, and bubbles do not rise.
8.0Fluid Flow
Conditions of Ideal Fluid Flow
Fluid is incompressible: density remains constant over time and position.
Fluid is non-viscous: no dissipative forces between fluid layers.
Flow is irrotational: fluid particles have zero angular velocity relative to each other.
Flow is steady (streamlined): flow properties do not change with time.
Steady (Streamline) Flow:
Velocity and density at any point remain constant with time.
Velocity and density may vary with position but not with time.
Streamlines:
Curves tangent to the fluid velocity direction at every point.
Streamline density is proportional to velocity magnitude.
Streamlines never cross (only one velocity direction at a point).
Flow Rates:
Mass flow rate dtdm=ρAV(mass per unit time).
Volume flow rate dtdV=AV (volume per unit time).
9.0Equation of Continuity
Continuity Equation:
This Represents the law of conservation of mass in fluid flow.
In steady flow, mass entering a tube per unit time = mass leaving it.
ρ1A1v1=ρ2A2v2(∵ρ1=ρ2)
A1v1=A2v2⇒Av=Constant
The velocity of liquid is smaller in the wider parts of a tube and larger in the narrower parts.
10.0Bernoulli's Theorem
In a steady, incompressible, and non-viscous flow of an ideal fluid, the total mechanical energy (sum of pressure energy, kinetic energy, and potential energy) per unit volume remains constant along a streamline.
Expresses the conservation of mechanical energy in fluid flow.
Venturimeter: A device used to measure the flow rate of a fluid through a pipe
The discharge or volume flow rate can be obtained as,
dtdV=A1v1=A1(A2A1)2−12gh
Speed of Efflux: Refers to the speed at which a fluid exits an orifice under pressure.
ρgh+P0=21ρv2+P0
v=2gh
Torricelli’s Theorem: The speed of a liquid flowing out of an orifice is equal to the speed it would gain if it fell freely from the liquid surface to the orifice.
Range
Vertical motion:
Time to fall from height (H−h),t=g2(H−h)
Horizontal motion:
Velocity of efflux: v=2gh
So Range, R=v⋅t=2ghg2(H−h)=2h(H−h)
Key Conclusions:
Symmetry Rh=RH−h(RangeissameforheighthandH−h)
Maximum RangedhdR=0⇒H−2h=0⇒h=2H, Range is maximum at h=2H
Maximum Range Value: Rmax=22H⋅2H
Time taken to empty a tank,t=aAg2H
12.0Surface Energy
Molecular Theory of Surface Tension
Molecules on the surface have extra energy due to unbalanced forces.
This energy per unit surface area is called surface energy.
Surface Tension and Work Done
A liquid film is formed on a wire frame with a movable wire of length l
Each surface pulls the wire with force Tl→total force from both surfaces=2Tl
To keep the wire in equilibrium, apply external force: F=2Tl
If the wire moves a small distance dx
Work donedW=F.dx=2Tl.dx=T(2l.dx)=T.dA
Increase in surface area: dA=2l.dx
T=dAdW
Surface tension T = work done per unit increase in surface area.
This work is stored as potential energy of the surface.
Splitting of a Bigger Drop into Smaller Droplets
Volume Conservation
Let a large drop of radius R split into n smaller drops of radius r
Work done (W) = Increase in surface energy: W=ΔE=4πT(nr2−R2)
Alternate Form of Work Done
Substitute r=n1/3R
W=4πR2T(n1/3−1)
W=4πR3T(r1−R1)
Thermal Effect
Energy used to increase surface area → internal energy decreases.
Temperature drops due to energy absorption.
Using heat relation: W=JmsΔθ
Final expression for temperature drop: Δθ=Jρp3T(r1−R1)
13.0Excess Pressure
Case
Number of Surfaces
Excess Pressure
Soap bubble (in air)
2
r4T
Liquid drop
1
r2T
Air bubble in liquid
(P1>P2)
1
r2T
14.0Contact Angle
Contact Angle and Surface Curvature
The surface of a liquid near its contact with another medium is usually curved.
The contact angle θC is the angle between the tangent to the liquid surface at the point of contact and the solid surface, measured inside the liquid.
Types of Molecular Forces
Cohesive force: Between molecules of the same substance (e.g., water-water).
Adhesive force: Between molecules of different substances (e.g., water-glass).
Contact Angle (θc)
Wettability
Example
θc<90o
Good wetting
Water on glass
θc=90o
Neutral
Water in a sipper vessel
θc>90o
Poor wetting (droplets)
Mercury on glass
15.0Capillary Rise
When a capillary tube is dipped in a liquid, the liquid rises due to surface tension.
The liquid forms a curved meniscus and rises until the upward surface tension force balances the weight of the liquid column.
Force Balance:
Upward force due to surface tension:F=2πrTcosθ
Downward force (weight of liquid column):W=πr2hρg
At equilibrium: 2πrTcosθ=πr2hρg⇒h=rρg2Tcosθ
Capillary Rise Formula: h=rρg2Tcosθ
16.0Viscosity
Viscosity is a fluid's property that opposes relative motion between its adjacent layers.
It is also called internal friction or fluid friction.
The tangential force resisting layer motion is called the viscous force.
Assumptions for Laminar Flow:
No-slip condition: Fluid in contact with a surface moves with the same velocity as the surface.
Velocity gradient:
Velocity increases uniformly from the bottom (stationary) layer to the top (moving) layer.
Each layer experiences a pull forward from the layer above and a drag backward from the one below.
This internal force results in laminar (smooth) flow.
Newton's Law of Viscosity
Coefficient of viscosity : η=RateofchangeofShearStrainShearStress⇒η=v/lF/A
Newtonian Fluids: Fvis=ηlAv
SI Units:m2N−s or deca poise
CGS Units:dyne−s/cm2 or poise (1 decapoise=10 poise)
Dimension:[M1L−1T−1]
Newtonian fluids
Non-Newtonian fluids
dydu=lv=Constant
Fvis=ηlAv
dydu is not constant
Fvis=ηdyAdu
dydu→Velocity Gradient
17.0Stoke's Law
A sphere of radius r, moving with velocity V relative to a fluid of viscosity η experiences a viscous drag, F=−6πηrV(FvisisoppositetoV)
18.0Terminal Velocity
Terminal velocity is the constant maximum velocity attained by an object falling through a fluid (like air or water), when the net force becomes zero.
v0=92ηr2g(ρL−ρs)
19.0Reynolds Number
Reynolds Number (Re)
Flow Type
Remarks
Re<1000
Laminar Flow
Smooth and orderly flow
Re>2000
Often Turbulent Flow
Irregular, chaotic fluid motion
1000<Re<2000
Transitional Flow
Flow may switch between laminar and turbulent
Reynolds number (Re) is a dimensionless quantity given by: Re=ηρvd
Critical speed: The velocity at which flow transitions from laminar to turbulent.
20.0Poiseuille’s Formula
Poiseuille studied fluid flow through narrow tubes (capillaries) and found.
Volume flow rate V is:
Directly proportional to pressure difference P
Directly proportional to r4 (fourth power of tube radius)
Inversely proportional to viscosity η
Inversely proportional to tube length l
Poiseuille’s Equation: V = Volume of liquid per second = V=8ηLπPr4
Liquid Resistance (R): R=πr48ηL so V=RP
21.0Combination of Tubes
Aspect
Series Combination
Parallel Combination
Pressure Difference
P=P1+P2
P1=P2=P
Flow Rate
V1=V2=V
(same through all tubes)
V=V1+V2
Flow Equation
V=R1+R2P
V=P(R11+R21)
Effective Resistance
Reff=R1+R2
Reff1=R11+R21
Resistance Formula(each tube)
R=πr48ηL
R=πr48ηL
Use When
Tubes are connected end-to-end
Tubes are connected side-by-side
Connection
Table of Contents
1.0Definition of Fluid
2.0Density of Liquid
3.0Pressure in a Fluid
4.0Variation of Pressure
5.0Pascal's Law
6.0Archimedes Principle
7.0Law of Floatation
8.0Fluid Flow
9.0Equation of Continuity
10.0Bernoulli's Theorem
11.0Applications of Bernoulli's Equation
12.0Surface Energy
13.0Excess Pressure
14.0Contact Angle
15.0Capillary Rise
16.0Viscosity
17.0Stoke's Law
18.0Terminal Velocity
19.0Reynolds Number
20.0Poiseuille’s Formula
20.0.1Volume flow rate V is:
21.0Combination of Tubes
Frequently Asked Questions
Cohesive forces: Forces between molecules of the same substance (e.g., water–water). Adhesive forces: Forces between molecules of different substances (e.g., water–glass).
Surface tension is the energy required to increase the surface area of a liquid. It causes the liquid surface to behave like a stretched elastic membrane.
Due to surface tension, liquids try to minimize surface area, and a sphere has the minimum surface area for a given volume.
In a steady, ideal fluid flow, the total energy at any point — which includes pressure energy, motion (kinetic) energy, and height (potential) energy — stays the same throughout the flow.ss
It relates pressure difference, viscosity, and radius of a capillary to the volume flow rate of liquid through it.