Concept Notes (Deep Explanation + Examples)
🔹 Introduction
In Hydraulics, understanding the properties of fluids is the foundation for solving all problems related to fluid statics, dynamics, and flow mechanics.
Fluids are substances that deform continuously under the action of shear stress, meaning they flow rather than retain shape — examples: water, oil, air, steam, etc.
In ECET and real-world applications like hydraulic brakes, turbines, pumps, and lubrication systems, knowing fluid properties helps in calculating pressure, flow rate, losses, and efficiency.
🔹 Basic Definition: What is a Fluid?
A fluid is a substance that can flow and does not resist deformation.
It includes both liquids and gases.
Unlike solids, fluids cannot sustain shear stress; they keep deforming until stress is removed.
🔹 Types of Fluids
| Type | Description | Example |
|---|---|---|
| Ideal Fluid | Incompressible & no viscosity | Theoretical concept |
| Real Fluid | Has viscosity & compressibility | Water, oil |
| Newtonian Fluid | Shear stress ∝ rate of shear strain | Water, air |
| Non-Newtonian Fluid | Shear stress ≠ rate of shear strain | Toothpaste, paints |
| Compressible Fluid | Density changes with pressure | Gases |
| Incompressible Fluid | Density nearly constant | Liquids |
🔹 Important Fluid Properties
Let’s go step-by-step through each property that is important for ECET 2026:
1️⃣ Density (ρ)
Density is the mass per unit volume of a fluid.
- SI Unit: kg/m³
- Example: Density of water at 4°C = 1000 kg/m³
Workshop Tip: In hydraulic systems, higher density fluid means more weight → affects pressure generation in cylinders.
2️⃣ Specific Weight (w or γ)
Specific weight = Weight per unit volume of fluid.
- Unit: N/m³
- Example: For water, γ = 9.81 × 1000 = 9810 N/m³
3️⃣ Specific Gravity (S or s)
It is the ratio of density of a fluid to density of water (at 4°C).
- No unit (dimensionless)
- Example: Oil has S = 0.8 → means oil is lighter than water.
4️⃣ Viscosity (μ)
Viscosity is the internal resistance of a fluid to flow.
For a Newtonian fluid:
Where:
τ = shear stress (N/m²),
μ = dynamic viscosity (N·s/m²),
du/dy = velocity gradient.
- Example: Honey has higher viscosity than water.
- Importance: In lubrication, higher viscosity means more frictional losses.
5️⃣ Kinematic Viscosity (ν)
- Unit: m²/s
- Example: If μ = 0.001 N·s/m² and ρ = 1000 kg/m³,
ν = 1 × 10⁻⁶ m²/s
6️⃣ Compressibility (β)
It measures how much a fluid’s volume decreases under pressure.
Where K = Bulk modulus of elasticity.
Liquids are almost incompressible, gases are highly compressible.
7️⃣ Vapor Pressure
It is the pressure exerted by vapor molecules on the liquid surface in equilibrium.
If local pressure drops below vapor pressure → Cavitation occurs (formation of vapor bubbles).
Example: Cavitation damages pump blades or turbine runners.
8️⃣ Surface Tension (σ)
It is the force per unit length acting along the surface of a liquid.
- Unit: N/m
- Example: Small insects walking on water surface.
9️⃣ Capillarity
Rise or fall of liquid in a small diameter tube due to surface tension.
- h = capillary rise/fall,
- θ = contact angle,
- d = tube diameter.
Example: Oil rises in a narrow tube; mercury falls due to adhesion > cohesion.
🔹 Practical Example (Workshop)
In a hydraulic lift, fluid properties decide performance:
- High density = more lifting force
- Correct viscosity = smooth piston movement
- Controlled compressibility = no jerks
- Low vapor pressure fluid avoids cavitation
🔹 ECET Shortcut Tips
- Always assume water as incompressible for ECET-level numericals.
- For gauge pressure, remember:
Viscosity decreases with temperature increase (for liquids).
Density decreases with temperature increase.
3️⃣ ⚙️ Formulas (Plain LaTeX)
4️⃣ 🔟 10 MCQs (GATE + ECET Mix)
- The property which defines mass per unit volume of a fluid is:
A) Specific weight
B) Density
C) Viscosity
D) Surface tension - The unit of dynamic viscosity in SI system is:
A) kg/m³
B) N·s/m²
C) m²/s
D) N/m - The ratio of viscosity to density is called:
A) Surface tension
B) Kinematic viscosity
C) Capillarity
D) Compressibility - If the density of water is 1000 kg/m³, then specific weight is approximately:
A) 100 N/m³
B) 9.81 N/m³
C) 9810 N/m³
D) 10000 N/m³ - A liquid has specific gravity 0.8. Its density is:
A) 800 kg/m³
B) 1000 kg/m³
C) 1250 kg/m³
D) 500 kg/m³ - Surface tension is measured in:
A) N/m
B) N/m²
C) N·s/m²
D) kg/m³ - The viscosity of liquids generally:
A) Increases with temperature
B) Decreases with temperature
C) Remains constant
D) First increases then decreases - The rise of liquid in a capillary tube is due to:
A) Adhesion
B) Cohesion
C) Surface tension
D) Compressibility - Which property is responsible for cavitation in pumps?
A) Density
B) Vapor pressure
C) Viscosity
D) Surface tension - The correct relation between pressure and bulk modulus is:
A)
B)
C)
D)
5️⃣ ✅ Answer Key
Q.No Answer
1 B
2 B
3 B
4 C
5 A
6 A
7 B
8 C
9 B
10 D
6️⃣ 🧠 MCQ Explanations
1️⃣ Density → B
Mass per unit volume defines density. Specific weight involves gravity, viscosity is resistance to flow.
2️⃣ Dynamic Viscosity → B
Measured in N·s/m² or Pa·s in SI. Example: water = 0.001 N·s/m².
3️⃣ Kinematic Viscosity → B
Defined as μ/ρ; units = m²/s.
4️⃣ Specific Weight → C
γ = ρg = 1000 × 9.81 = 9810 N/m³.
5️⃣ Specific Gravity → A
S = 0.8 ⇒ ρ = 0.8 × 1000 = 800 kg/m³.
6️⃣ Surface Tension → A
Force per unit length; hence unit N/m.
7️⃣ Viscosity Variation → B
In liquids, viscosity decreases as temperature increases due to weaker intermolecular forces.
8️⃣ Capillary Rise → C
Caused by surface tension and adhesive forces between liquid and wall.
9️⃣ Cavitation → B
Occurs when pressure falls below vapor pressure, forming vapor bubbles.
10️⃣ Compressibility Relation → D
Bulk modulus β = −(1/V)(dV/dp); negative sign due to volume reduction with increasing pressure.
7️⃣ 🎯 Motivation (ECET 2026 Specific)
“Properties of Fluids” is a frequently asked ECET topic because every question in hydraulics, fluid statics, and flow depends on these basic definitions.
Mastering this topic helps you solve 10+ marks worth of questions easily.
Keep revising formulas and units daily — consistency builds rank, not luck.
Small daily steps = ECET top rank. Stay focused and practice!
8️⃣ 📲 CTA
Join our ECET 2026 Mechanical WhatsApp Group for daily quizzes & study notes:
https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
