• NEET
      • Class 11th
      • Class 12th
      • Class 12th Plus
    • JEE
      • Class 11th
      • Class 12th
      • Class 12th Plus
    • Class 6-10
      • Class 6th
      • Class 7th
      • Class 8th
      • Class 9th
      • Class 10th
    • View All Options
      • Online Courses
      • Distance Learning
      • Hindi Medium Courses
      • International Olympiad
    • NEET
      • Class 11th
      • Class 12th
      • Class 12th Plus
    • JEE (Main+Advanced)
      • Class 11th
      • Class 12th
      • Class 12th Plus
    • JEE Main
      • Class 11th
      • Class 12th
      • Class 12th Plus
  • Classroom
    • NEET
      • 2025
      • 2024
      • 2023
      • 2022
    • JEE
      • 2025
      • 2024
      • 2023
      • 2022
    • Class 6-10
    • JEE Main
      • Previous Year Papers
      • Sample Papers
      • Result
      • Analysis
      • Syllabus
      • Exam Date
    • JEE Advanced
      • Previous Year Papers
      • Sample Papers
      • Mock Test
      • Result
      • Analysis
      • Syllabus
      • Exam Date
    • NEET
      • Previous Year Papers
      • Sample Papers
      • Mock Test
      • Result
      • Analysis
      • Syllabus
      • Exam Date
      • College Predictor
      • Counselling
    • NCERT Solutions
      • Class 6
      • Class 7
      • Class 8
      • Class 9
      • Class 10
      • Class 11
      • Class 12
    • CBSE
      • Notes
      • Sample Papers
      • Question Papers
    • Olympiad
      • NSO
      • IMO
      • NMTC
  • NEW
    • TALLENTEX
    • AOSAT
  • ALLEN E-Store
    • ALLEN for Schools
    • About ALLEN
    • Blogs
    • News
    • Careers
    • Request a call back
    • Book home demo
Home
JEE Physics
Fluid Mechanics

Fluid Mechanics

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.

Fluid mechanics

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

  • 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.

Gauge pressure = Absolute Pressure – Atmospheric 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

  1. Variation of Pressure at two points in horizontal plane

Variation of Pressure at two points in horizontal plane

PA​=PB​ 

The pressure is the same at two points in the same horizontal level.

  1. Variation of pressure at two points in different height depth

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

  1. Variation of pressure at two points when fluid is in horizontal acceleration

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.

  1. Variation of pressure at two points when fluid is in vertical 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.

  1. Variation of pressure when fluid is under both horizontal and vertical acceleration

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​
  1. Variation of pressure in a rotating fluid

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.

Variation of pressure in a rotating fluid

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 

Pascal's law - Hydraulic Lift

  • A small piston with area A1​​ applies force F1​​ on a confined fluid, creating pressure: P=A1​F1​​
  • This pressure is transmitted equally to a larger piston with area A2​​
  • Since pressure is the same in both pistons, A1​F1​​=A2​F2​​⇒F2=F1​A1​A2​​
  • 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

  1. At the same point in a fluid, pressure is the same in all directions. In the figure,

At the same point in a fluid, pressure is the same in all directions.

P1​=P2​=P3​=P4​

  1. Forces acting on a fluid in equilibrium have to be perpendicular to its surface. Because it cannot sustain the shear stress.
  2. In the same liquid pressure will be the same at all points at the same level. For example, in the figure:

In the same liquid pressure will be the same at all points at the same level.

ρ1​h1​=ρ2​h2​

h∝ρ1​

  1. 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.    

Torricelli Experiment (Barometer)

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.

  1. Manometer :It is a device used to measure the pressure of a gas inside a container. The U-shaped tube often contains mercury.

Manometer - a device used to measure the pressure of a gas inside a container.

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

Archimedes 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)=vi​ρL​g

vi​= immersed volume of solid, L=density of liquid, g=acceleration due to gravity

7.0Law of Floatation

Law 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⇒Vρs​​g=Vi​ρL​g⇒VVi​​=ρ​​Lρs​​
  • Percentage of Volume Immersed: VVi​​×100=ρL​ρs​​×100

Three Cases:

  1. ​ρs​<ρL​​ The object floats partially; only a fraction is submerged.
  2. ρs​=ρL​   The object is fully submerged but floats at any depth.
  3. 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)
  • Apparent weight: Wapp​=Wactual​−Upthrust=Vg(ρs​−ρL​)

If the liquid is water:

  • Relative Density (R.D) of the body: R.D=Loss in weight in waterWeight in air​=ρw​ρs​​

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=VρL​geff​, where geff​=g−a

Cases:

  1. Lift accelerating upward: geff​=g+a⇒Buoyant force increases
  2. Lift accelerating downward geff​=g−a⇒Buoyant force decreases
  3. 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

  1. Fluid is incompressible: density remains constant over time and position.
  2. Fluid is non-viscous: no dissipative forces between fluid layers.
  3. Flow is irrotational: fluid particles have zero angular velocity relative to each other.
  4. Flow is steady (streamlined): flow properties do not change with time.

Steady (Streamline) Flow:

Steady (Streamline) Flow

  • Velocity and density at any point remain constant with time.
  • Velocity and density may vary with position but not with time.

Streamline Flow

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).

Streamlines in a flow

Flow Rates:

Flow rates

  • Mass flow rate dtdm​=ρAV(mass per unit time).
  • Volume flow rate dtdV​=AV (volume per unit time).

9.0Equation of Continuity

Equation 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.

ρ1​A1​v1​=ρ2​A2​v2​    (∵ρ1​=ρ2​)

A1​v1​=A2​v2​⇒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

Bernoulli's Theorem

  1. 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.
  2. Expresses the conservation of mechanical energy in fluid flow.
  3. Applies to ideal fluids with these properties:
  • Incompressible
  • Non-viscous
  • Steady flow
  • Irrotational flow

Energy at any point in ideal flow includes:

  • Pressure energy
  • Kinetic energy
  • Potential energy

P1​+ρgh1​+21​ρv12​=P2​+ρgh2​+21​ρv22​⇒P+ρgh+21​ρv2=constant

11.0Applications of Bernoulli's Equation

  1. Venturimeter: A device used to measure the flow rate of a fluid through a pipe

Venturimeter

The discharge or volume flow rate can be obtained as,

dtdV​=A1​v1​=A1​(A2​A1​​)2−12gh​​

  1. Speed of Efflux: Refers to the speed at which a fluid exits an orifice under pressure.

Speed of Efflux

ρgh+P0​=21​ρv2+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 

Range in fluid mechanics

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=2gh​g2(H−h)​​=2h(H−h)​

Key Conclusions:

  1. Symmetry Rh​=RH−h​(Range is same for height h and H−h)
  2. Maximum Range dhdR​=0⇒H−2h=0⇒h=2H​, Range is maximum at h=2H​
  3. Maximum Range Value: Rmax​=22H​⋅2H​​

Time taken to empty a tank, t=aA​g2H​​

12.0Surface Energy

Surface 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 done dW=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

  1. Volume Conservation
  • Let a large drop of radius R  split into n smaller drops of radius r
  • Volume remains constant: 34​πR3=n⋅34​πr3⇒R3=nr3⇒r=n1/3R​
  1. Surface Area and Energy
  • Initial surface area: Ai​=4πR2
  • Final surface area: Af​=n⋅4πr2
  • Change in area: ΔA=4π(nr2−R2)
  • Initial surface energy: Ei​=4πR2T
  • Final surface energy: Ef​=4πTnr2
  • Work done (W) = Increase in surface energy: W=ΔE=4πT(nr2−R2)
  1. Alternate Form of Work Done

Substitute r=n1/3R​

W=4πR2T(n1/3−1)

W=4πR3T(r1​−R1​)

  1. 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)

Soap bubble (in air)


2

r4T​

Liquid drop

1

r2T​

Air bubble in liquid

Pressure in an air bubble in a 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

Types of Molecular Forces

15.0Capillary Rise

Capillary 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

  • 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:

  1. No-slip condition: Fluid in contact with a surface moves with the same velocity as the surface.
  2. 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

Newton's Law of Viscosity

Coefficient of viscosity : η=Rate of change of Shear StrainShear Stress​⇒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

Newtonian fluids

dydu​=lv​=Constant

Fvis​=ηlAv​

Non-Newtonian fluids

dydu​ is not constant 

Fvis​=ηdyAdu​

dydu​→Velocity Gradient

17.0Stoke's Law

Stoke's Law

A sphere of radius r, moving with velocity V relative to a fluid of viscosity η experiences a viscous drag, F=−6πηrV(Fvis​isoppositetoV)

Stoke's Law

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.

Terminal Velocity

v0​=92​ηr2g​(ρL​−ρs​)

Terminal Velocity graph

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:

  1. Directly proportional to pressure difference P
  2. Directly proportional to r4 (fourth power of tube radius)
  3. Inversely proportional to viscosity η
  4. 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​+R2​P​

V=P(R1​1​+R2​1​)

Effective Resistance

Reff​=R1​+R2​

Reff​1​=R1​1​+R2​1​

Resistance Formula (each tube)

R=πr48ηL​

R=πr48ηL​

Use When

Tubes are connected end-to-end

Tubes are connected side-by-side

Connection

Series Combination of tubes


Tubes in parallel


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.

Join ALLEN!

(Session 2025 - 26)


Choose class
Choose your goal
Preferred Mode
Choose State
  • About
    • About us
    • Blog
    • News
    • MyExam EduBlogs
    • Privacy policy
    • Public notice
    • Careers
    • Dhoni Inspires NEET Aspirants
    • Dhoni Inspires JEE Aspirants
  • Help & Support
    • Refund policy
    • Transfer policy
    • Terms & Conditions
    • Contact us
  • Popular goals
    • NEET Coaching
    • JEE Coaching
    • 6th to 10th
  • Courses
    • Online Courses
    • Distance Learning
    • Online Test Series
    • International Olympiads Online Course
    • NEET Test Series
    • JEE Test Series
    • JEE Main Test Series
  • Centers
    • Kota
    • Bangalore
    • Indore
    • Delhi
    • More centres
  • Exam information
    • JEE Main
    • JEE Advanced
    • NEET UG
    • CBSE
    • NCERT Solutions
    • Olympiad
    • NEET 2025 Results
    • NEET 2025 Answer Key
    • NEET College Predictor
    • NEET 2025 Counselling

ALLEN Career Institute Pvt. Ltd. © All Rights Reserved.

ISO