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Fluid Mechanics practice questions

10 free Fluid Mechanics questions for the FE Mechanical Exam (FE) exam, each with the correct answer and an explanation for every option.

Start the questionsFree 10-question FE quiz

What this domain covers

One of the four largest domains: fluid properties and statics, energy and momentum in flow, pipe and external flow, and turbomachinery.

Exam weight
10%
On a real exam
≈ 11 questions
In our bank
60 questions

Topics that come up most

  • Hydrostatic pressure and buoyancy
  • Bernoulli and energy equation with losses
  • Pipe friction and the Moody diagram
  • Momentum forces on vanes and bends
  • Pump and fan performance and similarity laws

How to study it

Practise full pipe-flow problems from Reynolds number to friction factor to head loss; that chain appears again and again.

Sample Fluid Mechanics questions

Try each one before opening the answer. Original questions written to the official exam objectives.

  1. Question 1

    A pipe has a roughness of 0.05 mm and a diameter of 100 mm. Its relative roughness is:

    • A2,000
    • B0.05
    • C0.0005
    • D0.005
    Show answer and explanation

    Answer: C) 0.0005

    • AIncorrect. That's inverted.
    • BIncorrect. Divide by the diameter.
    • CCorrect. ε/D = 0.05/100.
    • DIncorrect. Recheck the division.

  2. Question 2

    A venturi has a 0.01-m² throat, β = 0.5 and a 20-kPa pressure drop with water. The ideal flow rate is about:

    • A0.020 m³/s
    • B0.063 m³/s
    • C0.200 m³/s
    • D0.065 m³/s
    Show answer and explanation

    Answer: D) 0.065 m³/s

    • AIncorrect. Recheck the calculation.
    • BIncorrect. Include the \(1 - \beta^4\) term.
    • CIncorrect. Take the square root.
    • DCorrect. \(Q = A_2\sqrt{2\Delta p/[\rho(1 - \beta^4)]} = 0.01(6.53)\).

  3. Question 3

    A hydraulic system delivers 0.01 m³/s at a pressure rise of 2 MPa. The hydraulic power is:

    • A2 kW
    • B200 kW
    • C20 kW
    • D0.2 kW
    Show answer and explanation

    Answer: C) 20 kW

    • AIncorrect. That's off by a factor of 10.
    • BIncorrect. That's off by a factor of 10.
    • CCorrect. \(P = \Delta p \times Q\).
    • DIncorrect. That's off by a factor of 100.

  4. Question 4

    Air (cp = 1,005 J/kg·K) at 300 K flows at 200 m/s. Its stagnation temperature is about:

    • A320 K
    • B340 K
    • C300 K
    • D500 K
    Show answer and explanation

    Answer: A) 320 K

    • ACorrect. \(T_0 = T + V^2/(2c_p) = 300 + 19.9\).
    • BIncorrect. Divide by 2cp.
    • CIncorrect. Add the kinetic energy term.
    • DIncorrect. Recheck the calculation.

  5. Question 5

    The gage pressure at 10 m depth in seawater (SG = 1.03) is about:

    • A202 kPa
    • B98 kPa
    • C10.3 kPa
    • D101 kPa
    Show answer and explanation

    Answer: D) 101 kPa

    • AIncorrect. That would add atmospheric pressure.
    • BIncorrect. That's fresh water.
    • CIncorrect. That's for 1 m.
    • DCorrect. \(1.03(9.81)(10)\).

  6. Question 6

    A valve at the downstream end of a long water pipeline is closed suddenly while water is flowing. Immediately upstream of the valve, the pressure:

    • ARemains essentially unchanged, because water is treated as incompressible
    • BRises by about ρV²/2, as the steady-flow Bernoulli equation predicts
    • CRises sharply as a pressure wave travels upstream at roughly the acoustic speed
    • DFalls gradually, because friction losses grow as the flow decelerates
    Show answer and explanation

    Answer: C) Rises sharply as a pressure wave travels upstream at roughly the acoustic speed

    Sudden valve closure causes water hammer: the decelerating fluid column produces a pressure surge of about ρcΔV that travels through the pipe at roughly the speed of sound in the fluid. This surge is much larger than steady-flow Bernoulli would predict.

    • AIncorrect. Water is only slightly compressible, but that small compressibility, together with the pipe wall's elasticity, is exactly what lets a pressure wave form and travel along the pipe. The pressure does not stay constant.
    • BIncorrect. Steady-flow Bernoulli doesn't apply to a sudden transient. The real surge is about ρcΔV, where c is the wave speed (hundreds to over a thousand m/s). That is far larger than the velocity head term.
    • CCorrect. This is water hammer. Suddenly stopping the moving fluid column produces a pressure rise of about ρcΔV, which travels upstream as a wave and reflects back and forth along the pipe.
    • DIncorrect. Friction losses shrink, not grow, as the flow slows. Upstream of the valve, the momentum of the stopping fluid column produces a sharp pressure rise, not a gradual drop.

  7. Question 7

    Water (ρ = 1,000 kg/m³, μ = 0.001 Pa·s) flows at 2 m/s in a 50-mm pipe. The Reynolds number is:

    • A10,000
    • B100,000
    • C1,000
    • D1,000,000
    Show answer and explanation

    Answer: B) 100,000

    • AIncorrect. Recheck the powers of ten.
    • BCorrect. \(Re = \rho VD/\mu = 1{,}000(2)(0.05)/0.001\).
    • CIncorrect. Recheck the powers of ten.
    • DIncorrect. Recheck the diameter in meters.

  8. Question 8

    For a sphere in very slow (creeping) flow, the drag coefficient is approximately:

    • A24/Re
    • B0.47, a constant
    • C64/Re
    • DRe/24
    Show answer and explanation

    Answer: A) 24/Re

    • ACorrect. This is Stokes' law.
    • BIncorrect. That's for higher Reynolds numbers.
    • CIncorrect. That's the laminar pipe friction factor.
    • DIncorrect. That's inverted.

  9. Question 9

    A hydro turbine takes 2 m³/s at a 50-m net head with 90% efficiency. Its power output is about:

    • A981 kW
    • B883 kW
    • C1,090 kW
    • D90 kW
    Show answer and explanation

    Answer: B) 883 kW

    • AIncorrect. Apply the efficiency.
    • BCorrect. \(P = \eta\gamma QH = 0.9(9.81)(2)(50)\).
    • CIncorrect. Multiply by efficiency, don't divide.
    • DIncorrect. Include the unit weight.

  10. Question 10

    For steady compressible flow in a duct, conservation of mass requires constant:

    • AρAV
    • BAV only
    • CρV only
    • DpV
    Show answer and explanation

    Answer: A) ρAV

    • ACorrect. The mass flow rate.
    • BIncorrect. That holds only for incompressible flow.
    • CIncorrect. Include the area.
    • DIncorrect. Mass flow is ρAV.

Fluid Mechanics: FAQs

How much of the FE exam is Fluid Mechanics?

Fluid Mechanics is 10% of the FE exam by the official exam guide's weighting, which on a 110-question form works out to roughly 11 questions. The real exam's exact split varies by form.

What does the Fluid Mechanics domain cover?

One of the four largest domains: fluid properties and statics, energy and momentum in flow, pipe and external flow, and turbomachinery.

How should I study Fluid Mechanics for FE?

Practise full pipe-flow problems from Reynolds number to friction factor to head loss; that chain appears again and again.

Where can I practise more Fluid Mechanics questions?

This page has 10 free Fluid Mechanics questions with answers. The ExamWize FE question bank has 60 questions in this domain. Every practice exam result breaks your score down by domain, and Review Mode re-drills the questions you missed.

Other FE topics

Also: 15 free questions across all topics, the FE cheat sheet and the study guide.

Get Fluid Mechanics exam-ready

The full FE bank has 60 Fluid Mechanics questions. Timed practice exams match the real exam's length and domain weights, every result shows your score per domain, and Review Mode re-drills the questions you missed.

See the full FE practice examsTake the free diagnostic

Not affiliated with NCEES; questions are original, written from the official exam outline, and never copied from the real exam.