Fluid Mechanics MCQ With Answers for JKSSB Junior Engineer (Mechanical)

Technical · 2026-10-21 · 6 min read

Fluid mechanics and hydraulic machines make up a large block of the mechanical technical paper. Expect questions on fluid properties, Bernoulli's equation, flow measurement, pumps and turbines. This page gives a revision list and then solved MCQs with explanations.

Quick revision list

  • Properties: density of water 1,000 kg/m³, dynamic viscosity in Pa·s, and Newton's law of viscosity
  • Statics: Pascal's law, hydrostatic pressure ρgh and Archimedes' principle
  • Flow: continuity equation, Bernoulli's equation, Darcy–Weisbach head loss and the Mach number
  • Measurement: venturimeter, Pitot tube and manometers
  • Machines: Pelton (impulse, high head), Francis (reaction, medium head), Kaplan (low head), centrifugal pumps, priming and cavitation

Worked numerical problems

  • Continuity: water flows at 2 m/s in a pipe of 20 cm diameter that narrows to 10 cm. The area falls to one quarter, so the velocity is 8 m/s
  • Discharge: a pipe of diameter 0.1 m carries water at 3 m/s. The area is 0.007854 m², so Q = A × v = 0.0236 m³/s (about 23.6 litres per second)
  • Hydrostatic pressure: at a depth of 5 m in water, the gauge pressure is ρgh = 1,000 × 9.81 × 5 = 49,050 Pa
  • Buoyancy: a block of volume 0.2 m³ fully immersed in water feels an upward force of 1,000 × 9.81 × 0.2 = 1,962 N
  • Reynolds number: water at 2 m/s in a 0.05 m pipe, with viscosity 0.001 Pa·s, has Re = ρvD ÷ μ = 1,000 × 2 × 0.05 ÷ 0.001 = 100,000, so the flow is turbulent
  • Pump power: a pump delivers 0.02 m³/s against a head of 20 m with an efficiency of 70%. The input power is ρgQH ÷ η = 1,000 × 9.81 × 0.02 × 20 ÷ 0.7 ≈ 5.6 kW

Bernoulli's equation in use

Bernoulli's equation states that p/ρg + v²/2g + z is constant along a streamline for steady, incompressible, frictionless flow. Each term is a head, measured in metres of liquid: pressure head, velocity head and elevation head. A venturimeter uses the drop in pressure as the fluid speeds up in the throat. The discharge is Q = C_d × A₁A₂ × √(2gh) ÷ √(A₁² − A₂²), where C_d is the coefficient of discharge, which is slightly less than 1.

Choosing a turbine

  • Pelton wheel: an impulse turbine for high heads, above about 250 m, and low discharge
  • Francis turbine: a reaction turbine for medium heads, about 40 to 250 m
  • Kaplan turbine: an axial-flow reaction turbine for low heads, below about 40 m, and large discharge
  • Specific speed: it rises from the Pelton wheel to the Kaplan turbine, so a higher specific speed means a lower head and more flow

Pump terms

Priming fills the casing with liquid before starting. Cavitation occurs when the local pressure falls below the vapour pressure, forming bubbles that collapse and pit the metal. The net positive suction head (NPSH) available must exceed the NPSH required to avoid it. A positive displacement pump gives a nearly constant discharge, while a centrifugal pump's discharge changes with the head.

Common mistakes

  • Using the diameter instead of the area in the continuity equation
  • Forgetting that velocity scales with the inverse of the area, so halving the diameter quadruples the velocity
  • Mixing gauge and absolute pressure
  • Using the wrong turbine for the head given in the question

Questions candidates ask

Which formula do I use for discharge? Q = A × v, with area in square metres and velocity in metres per second.

What is the difference between laminar and turbulent flow? Laminar flow is smooth and in layers, with a low Reynolds number. Turbulent flow is irregular and has a high Reynolds number.

Why does cavitation damage a pump? Bubbles form where the pressure is low and collapse near the metal, pitting it.

Practise

Solve the MCQs below, then try the full-length mocks and read the exam guide. Strength of materials is next: see the strength of materials MCQs.

By Shivam · Written with AI assistance and fact-checked · Questions on this page are original · Editorial policy · Report an error

Practice questions

Q1. The density of water at normal temperature is about:

  1. 1,000 kg/m³
  2. 100 kg/m³
  3. 13,600 kg/m³
  4. 10 kg/m³
Show answer

1,000 kg/m³ — Water has a density of 1,000 kg/m³ (1 g/cm³).

Q2. The SI unit of dynamic viscosity is:

  1. kg/m³
  2. N/m
  3. m²/s
  4. Pa·s
Show answer

Pa·s — Dynamic viscosity is measured in pascal-seconds. Kinematic viscosity is in m²/s.

Q3. Newton's law of viscosity states that shear stress is proportional to:

  1. The density
  2. The velocity gradient
  3. The temperature
  4. The pressure
Show answer

The velocity gradient — τ = μ (du/dy).

Q4. Pascal's law states that pressure applied to a confined fluid is:

  1. Transmitted equally in all directions
  2. Doubled at the bottom
  3. Transmitted only downwards
  4. Lost through the walls
Show answer

Transmitted equally in all directions — This is the principle behind the hydraulic jack and press.

Q5. Bernoulli's equation applies along a streamline for flow that is:

  1. Turbulent with high friction
  2. Unsteady and compressible
  3. Steady, incompressible and frictionless
  4. Rotational with heat transfer
Show answer

Steady, incompressible and frictionless — It states that the sum of pressure, kinetic and potential heads is constant under those conditions.

Q6. A venturimeter is used to measure:

  1. Viscosity
  2. Temperature of the fluid
  3. Pressure in a tank
  4. The flow rate through a pipe
Show answer

The flow rate through a pipe — It uses the pressure drop across a converging throat.

Q7. A Pitot tube is used to measure:

  1. The discharge of a pump
  2. The viscosity
  3. The velocity of flow at a point
  4. The level in a tank
Show answer

The velocity of flow at a point — It senses the stagnation pressure to find velocity.

Q8. Flow through a pipe is generally laminar when the Reynolds number is:

  1. Exactly 10,000
  2. Below about 2,000
  3. Above 4,000
  4. Equal to the Mach number
Show answer

Below about 2,000 — Pipe flow is laminar below about 2,000 and turbulent above about 4,000.

Q9. The Reynolds number is the ratio of:

  1. Viscous forces to inertia forces
  2. Gravity forces to inertia forces
  3. Pressure forces to surface tension
  4. Inertia forces to viscous forces
Show answer

Inertia forces to viscous forces — Re = ρVD/μ.

Q10. The pressure at a depth h in a static liquid of density ρ is:

  1. ρg/h
  2. ρgh (gauge)
  3. gh/ρ
  4. ρh/g
Show answer

ρgh (gauge) — Hydrostatic pressure grows linearly with depth.

Q11. The gauge pressure at a depth of 10 m in water is about:

  1. 10 kPa
  2. 9.8 kPa
  3. 98 kPa
  4. 980 kPa
Show answer

98 kPa — p = ρgh = 1,000 × 9.81 × 10 ≈ 98,100 Pa.

Q12. Archimedes' principle states that the buoyant force equals:

  1. The weight of the body
  2. The weight of the fluid displaced
  3. The volume of the body
  4. The density of the fluid
Show answer

The weight of the fluid displaced — A body immersed in a fluid loses weight equal to the weight of fluid it displaces.

Q13. A floating body is in stable equilibrium when its metacentre lies:

  1. At the bottom of the body
  2. Outside the fluid
  3. Above the centre of gravity
  4. Below the centre of gravity
Show answer

Above the centre of gravity — A positive metacentric height gives a restoring moment.

Q14. For incompressible flow, the continuity equation is:

  1. A1V1 = A2V2
  2. p1V1 = p2V2
  3. ρ1 = A1V1
  4. V1/A1 = V2/A2
Show answer

A1V1 = A2V2 — The discharge is the same at every section.

Q15. If the diameter of a pipe is halved at a contraction, the velocity of the water:

  1. Becomes four times
  2. Becomes half
  3. Becomes twice
  4. Stays the same
Show answer

Becomes four times — Area falls to a quarter, so velocity rises fourfold.

Q16. The Darcy–Weisbach equation for head loss in a pipe is:

  1. hf = f·L·V / D
  2. hf = f·D·V² / L
  3. hf = 4fLV² / g
  4. hf = f·L·V² / (2gD)
Show answer

hf = f·L·V² / (2gD) — Head loss grows with length and the square of velocity, and falls with diameter.

Q17. The Mach number is the ratio of:

  1. Flow speed to the speed of sound
  2. Inertia force to viscous force
  3. Pressure to density
  4. Speed of sound to flow speed
Show answer

Flow speed to the speed of sound — Flow with Mach number above 1 is supersonic.

Q18. A Pelton wheel is a:

  1. Reaction turbine suited to low heads
  2. Impulse turbine suited to high heads
  3. Centrifugal pump
  4. Axial flow turbine for very low heads
Show answer

Impulse turbine suited to high heads — Water jets strike the buckets at atmospheric pressure.

Q19. The Kaplan turbine is suited to:

  1. Steam power plants
  2. Medium heads only
  3. Very high heads
  4. Low heads and large discharge
Show answer

Low heads and large discharge — It is an axial-flow reaction turbine with adjustable blades.

Q20. The Francis turbine is a:

  1. Positive displacement pump
  2. Gas turbine
  3. Reaction turbine for medium heads
  4. Impulse turbine for high heads
Show answer

Reaction turbine for medium heads — Water enters radially and leaves axially.

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