Fluid Mechanics and Hydraulics MCQ With Answers for JKSSB Junior Engineer (Civil)

Technical · 2026-10-27 · 6 min read

Fluid mechanics and hydraulics carried 15 marks in the board's earlier Junior Engineer (Civil) syllabus, which names pipe flow, open channels, Bernoulli's equation, flow measurement, hydraulic pumps and the hydraulic jump. Our guide to the 2026 notice explains how that syllabus fits the new recruitment. This page gives a revision list, worked problems and solved MCQs.

Quick revision list

  • Properties: density of water 1,000 kg/m³, specific weight 9.81 kN/m³, dynamic viscosity in Pa·s
  • Statics: pressure p = ρgh, force on a submerged plane = ρg × area × depth of the centroid, and the centre of pressure lies below the centroid
  • Flow: continuity A₁v₁ = A₂v₂, Bernoulli's equation, and the momentum equation
  • Pipes: Darcy–Weisbach hf = fLv² ÷ 2gD, with f = 64 ÷ Re for laminar flow, losses in series and parallel, and maximum power transmission when friction loss is one-third of the supply head
  • Open channels: Chezy v = C√(RS), Manning v = (1 ÷ n) × R^(2/3) × S^(1/2), hydraulic radius R = A ÷ P, specific energy, critical depth and the hydraulic jump
  • Measurement: venturimeter, orifice, Pitot tube, rectangular weir and V-notch
  • Machines: centrifugal pumps, priming, cavitation and NPSH

Worked numerical problems

  • Continuity: water flows at 1.5 m/s in a 30 cm pipe that narrows to 15 cm. The area falls to one quarter, so the velocity becomes 6 m/s
  • Force on a gate: a vertical gate 2 m wide and 3 m high has water up to its top. F = 1,000 × 9.81 × (2 × 3) × 1.5 = 88.29 kN, acting 2 m below the surface (two-thirds of the depth)
  • Pipe friction: D = 0.2 m, L = 100 m, v = 2 m/s and f = 0.02 give hf = 0.02 × 100 × 4 ÷ (2 × 9.81 × 0.2) = 2.04 m
  • Manning's formula: a rectangular channel 3 m wide and 1 m deep, with n = 0.015 and slope 0.0004, has R = 3 ÷ 5 = 0.6 m. Then v = (1 ÷ 0.015) × 0.6^(2/3) × 0.02 = 0.95 m/s and Q = 3 × 0.95 = 2.85 m³/s
  • Critical depth: Q = 12 m³/s in a 4 m wide rectangular channel gives q = 3 m²/s, so yc = (q² ÷ g)^(1/3) = (9 ÷ 9.81)^(1/3) = 0.97 m
  • Weir discharge: Q = (2/3) × C_d × L × √(2g) × H^(3/2) with C_d = 0.6, L = 2 m and H = 0.3 m gives 0.58 m³/s

Open channel flow in brief

Specific energy is E = y + v² ÷ 2g, the energy per unit weight measured from the channel bed. For a given discharge it has a minimum at the critical depth, and in a rectangular channel the minimum is 1.5 times the critical depth. A depth above critical means subcritical flow, which is slow and deep, and a depth below critical means supercritical flow, which is fast and shallow. When supercritical flow meets a deeper, slower stream it jumps up to the conjugate depth, losing energy as turbulence. This is the hydraulic jump, which engineers use below spillways to dissipate energy.

Pipe flow and pumps in brief

Head loss in a pipe has a friction part, given by the Darcy–Weisbach formula, and minor losses at bends, valves and entrances. For two reservoirs joined by a long pipe the loss at the entry and exit is small beside the friction loss. A centrifugal pump must be primed, which means the casing and suction pipe are filled with liquid, because it cannot lift water on its own when full of air. If the local pressure falls to the vapour pressure of the liquid, bubbles form and collapse, and this cavitation damages the impeller. Keeping the available NPSH above the pump's required NPSH avoids it.

Common mistakes

  • Working out the hydraulic radius R = A ÷ P with the free surface counted as part of the wetted perimeter, when only the surfaces touching the channel count
  • Mixing up the exponents in Manning's formula (R to the power 2/3 and S to the power 1/2)
  • Forgetting that discharge over a rectangular weir goes with H^(3/2) and over a V-notch with H^(5/2)
  • Treating a hydraulic jump as flow going from subcritical to supercritical, when it is the opposite

Practise

Work the solved questions below, then read the Junior Engineer (Civil) guide for the dates and the marks for every subject. Other subject pages in this series cover mechanics and structures, RCC, and construction management.

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

Practice questions

Q1. The gauge pressure at depth h in a liquid of density ρ is:

  1. ρh ÷ g
  2. ρgh
  3. ρg ÷ h
  4. ρgh²
Show answer

ρgh — Pressure in a static liquid rises linearly with depth as p = ρgh.

Q2. The specific weight of water is about:

  1. 9.81 kN/m³
  2. 1 kN/m³
  3. 98.1 kN/m³
  4. 0.981 kN/m³
Show answer

9.81 kN/m³ — Specific weight is ρg, which is 1,000 × 9.81 N/m³ for water.

Q3. A pressure of 98.1 kPa is equal to a water head of:

  1. 1 m
  2. 100 m
  3. 9.81 m
  4. 10 m
Show answer

10 m — Head = pressure ÷ specific weight = 98.1 ÷ 9.81 = 10 m.

Q4. The SI unit of dynamic viscosity is:

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

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

Q5. The Reynolds number is the ratio of:

  1. Gravity forces to inertial forces
  2. Inertial forces to viscous forces
  3. Pressure forces to viscous forces
  4. Surface tension to inertial forces
Show answer

Inertial forces to viscous forces — Re = ρvD ÷ μ compares inertia with viscosity.

Q6. Flow in a circular pipe is generally laminar when the Reynolds number is below about:

  1. 2,000
  2. 4,000
  3. 10,000
  4. 500,000
Show answer

2,000 — Below about 2,000 the flow is laminar, and above about 4,000 it is turbulent.

Q7. For laminar flow in a circular pipe the Darcy friction factor is:

  1. 0.316 ÷ Re^0.25
  2. Re ÷ 64
  3. 1 ÷ Re
  4. 64 ÷ Re
Show answer

64 ÷ Re — f = 64 ÷ Re for laminar flow. The 0.316 ÷ Re^0.25 form is the Blasius formula for smooth turbulent pipes.

Q8. For incompressible flow in a pipe of varying area, the continuity equation gives:

  1. v₁ ÷ A₁ = v₂ ÷ A₂
  2. A₁ + v₁ = A₂ + v₂
  3. A₁v₁ = A₂v₂
  4. A₁v₂ = A₂v₁
Show answer

A₁v₁ = A₂v₂ — The same volume flow passes every section, so area times velocity is constant.

Q9. In Bernoulli's equation the term v² ÷ 2g is called the:

  1. Pressure head
  2. Velocity head
  3. Datum head
  4. Friction head
Show answer

Velocity head — Each term has units of length. v² ÷ 2g is the velocity head.

Q10. Bernoulli's equation expresses the conservation of:

  1. Energy
  2. Mass
  3. Momentum
  4. Charge
Show answer

Energy — It is the energy equation for an ideal fluid along a streamline.

Q11. A venturimeter is used to measure:

  1. The viscosity of a liquid
  2. The density of a liquid
  3. The temperature of the flow
  4. The discharge through a pipe
Show answer

The discharge through a pipe — The pressure drop between the inlet and the throat gives the discharge.

Q12. A Pitot tube measures:

  1. The discharge directly
  2. The viscosity
  3. The velocity of flow at a point
  4. The depth of a channel
Show answer

The velocity of flow at a point — It senses the stagnation pressure, from which the point velocity is found.

Q13. The centre of pressure on a submerged vertical plane lies:

  1. At the centroid
  2. Below the centroid
  3. Above the centroid
  4. At the free surface
Show answer

Below the centroid — Pressure increases with depth, so the resultant acts below the centroid.

Q14. The hydraulic radius of a channel section is:

  1. Area ÷ wetted perimeter
  2. Wetted perimeter ÷ area
  3. Top width ÷ depth
  4. Area ÷ top width
Show answer

Area ÷ wetted perimeter — R = A ÷ P.

Q15. Chezy's formula for the velocity in an open channel is:

  1. v = C × R × S
  2. v = C√(R ÷ S)
  3. v = C × S√R ÷ 2
  4. v = C√(RS)
Show answer

v = C√(RS) — C is the Chezy coefficient, R the hydraulic radius and S the bed slope.

Q16. Manning's formula for velocity is:

  1. v = (1 ÷ n) × R^(1/2) × S^(2/3)
  2. v = n × R^(2/3) × S^(1/2)
  3. v = (1 ÷ n) × R^(2/3) × S^(1/2)
  4. v = (1 ÷ n) × R^(3/2) × S^(1/2)
Show answer

v = (1 ÷ n) × R^(2/3) × S^(1/2) — n is the roughness coefficient, R the hydraulic radius and S the slope.

Q17. Flow in an open channel is critical when the Froude number is:

  1. Equal to 0
  2. Equal to 1
  3. Greater than 1
  4. Equal to 2
Show answer

Equal to 1 — Fr = v ÷ √(gy). At critical flow the flow velocity equals the speed of a small surface wave.

Q18. A hydraulic jump forms when the flow changes from:

  1. Supercritical to subcritical
  2. Subcritical to supercritical
  3. Laminar to turbulent
  4. Steady to unsteady
Show answer

Supercritical to subcritical — Fast, shallow flow slows suddenly into a deeper stream, with a loss of energy.

Q19. The most economical rectangular channel section has:

  1. Width equal to the depth
  2. Width equal to three times the depth
  3. Depth equal to twice the width
  4. Width equal to twice the depth
Show answer

Width equal to twice the depth — For a given area it gives the least wetted perimeter when b = 2y.

Q20. The minimum specific energy in a rectangular channel is:

  1. Equal to the critical depth
  2. Twice the critical depth
  3. 1.5 times the critical depth
  4. 0.5 times the critical depth
Show answer

1.5 times the critical depth — At critical flow the velocity head is half the depth, so Emin = yc + yc ÷ 2 = 1.5 yc.

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