FE Civil Fluid Mechanics practice problems
6 free FE Civil Fluid Mechanics practice problems with answers and full worked solutions. Fluid Mechanics carries 6–9 of the 110 questions on the FE Civil exam (NCEES specification effective July 2020). Pick an answer to check it, then read the worked solution and why each wrong choice is wrong.
6 worked Fluid Mechanics problems
Question 1 of 6
Water moves at 4.5 ft/s through a 10-in diameter pipe. The volumetric flow rate is most nearly:
Water moving at 4.5 ft/s through a pipe of 10 in inside diameter. Show the answer and the reasoning
Why A
Given: V = 4.5 ft/s in a 10-in pipe, D = 0.833 ft. Relation: Q = VA, with A = piD^2/4 in ft^2. Substitute: A = 0.5450 ft^2; Q = 4.5 x 0.5450. Result: Q = 2.452 cfs. Check: the diameter goes into feet before it is squared.
Why B is tempting
Divided by the area instead of multiplying.
Why C is tempting
Used piD^2 rather than piD^2/4 for the pipe area.
Why D is tempting
Used the circumference in place of the flow area.
Question 2 of 6
A fluid has a specific weight of 100 lb/ft^3. Its density is most nearly:
Show the answer and the reasoning
Why C
Given: specific weight 100 lb/ft^3. Relation: gamma = rho g, so rho = gamma/g. Substitute: rho = 100/32.2. Result: rho = 3.106 slug/ft^3.
Why A is tempting
Inverted the ratio, giving an acceleration per unit specific weight rather than a density.
Why B is tempting
Divided by the specific weight of water, which gives specific gravity rather than density.
Why D is tempting
Reported the specific weight.
Question 3 of 6
A tank holds a liquid of specific gravity 0.8. The gauge pressure at a depth of 72 ft is most nearly:
Open tank holding a liquid of specific gravity 0.8, with a point marked 72 ft below the free surface. Show the answer and the reasoning
Why A
Given: SG = 0.8, depth 72 ft below the free surface. Relation: gauge pressure p = SG gamma_w h, divided by 144 to give psi. Substitute: p = 0.8(62.4)(72)/144. Result: p = 24.96 psi.
Why B is tempting
Used the specific weight of water, ignoring the fluid specific gravity.
Why C is tempting
Added atmospheric pressure. The question asks for gauge, not absolute, pressure.
Why D is tempting
Divided by 12 rather than 144.
Question 4 of 6
Water flows at 4 cfs through a head of 170 ft. The hydraulic power is most nearly:
Show the answer and the reasoning
Why D
Given: Q = 4 cfs through a head of 170 ft. Relation: P = gamma Q h, divided by 550 ft-lb/s per horsepower. Substitute: P = 62.4(4)(170)/550. Result: P = 77.15 hp.
Why A is tempting
Used 33,000 ft-lb/min. With the discharge in cubic feet per second the power is per second, so the constant is 550.
Why B is tempting
Used 8.34 lb per gallon with a discharge in cubic feet per second.
Why C is tempting
Multiplied by 0.746, converting the horsepower to kilowatts.
Question 5 of 6
A pitot-static tube in a water pipe registers a differential head of 4.2 ft. The local velocity is most nearly:
Pitot-static tube facing the flow in a water pipe, registering a differential head of 4.2 ft between its stagnation and static taps. Show the answer and the reasoning
Why C
Given: a differential (stagnation minus static) head of 4.2 ft of water. Relation: the stagnation point turns velocity head into pressure head, so dh = V^2/2g and V = sqrt(2g dh). Substitute: V = sqrt(2 x 32.2 x 4.2). Result: V = 16.45 ft/s.
Why A is tempting
Left the acceleration of gravity out.
Why B is tempting
Halved the velocity.
Why D is tempting
Left the head outside the radical.
Question 6 of 6
A liquid has a specific gravity of 0.92. Its specific weight is most nearly:
Show the answer and the reasoning
Why B
Given: specific gravity 0.92. Relation: gamma = SG x gamma_water, with gamma_water = 62.4 lb/ft^3. Substitute: gamma = 0.92(62.4). Result: gamma = 57.41 lb/ft^3. Check: SG is a ratio, so multiplying by the specific weight of water restores the units.
Why A is tempting
Multiplied by the acceleration of gravity rather than the specific weight of water.
Why C is tempting
Used the SI density of water in a US customary calculation.
Why D is tempting
Applied gravity a second time; the specific weight of water already includes it.
Going further
Frequently asked questions
How many Fluid Mechanics questions are on the FE Civil exam?
Between 6 and 9 of the 110 questions, according to the NCEES FE Civil specification effective July 2020.
Are these real NCEES exam questions?
No. NCEES does not release its exam questions. These are original FE-style problems written to the NCEES specification and the FE Reference Handbook; every calculated key is checked by two independent solution methods.
Are these Fluid Mechanics practice problems free?
Yes. All 6 problems on this page, with answers and full worked solutions, are free and need no account.
Check where you stand, free
The quick diagnostic is eight FE Civil questions in about ten minutes, with no account. The free mock is a full 110-question, 5 hour 20 minute FE Civil paper with a worked solution for every question.