Hydrostatic Pressure practice problems for the FE Civil exam

Hydrostatic pressure is the first result in fluid statics and the one the FE Civil exam returns to most often, because tanks, dams, retaining structures, manometers, and buried pipes all reduce to it. The pressure at a point in a fluid at rest depends only on the specific weight of the fluid and the vertical depth below the free surface. It does not depend on the shape of the container, the total volume of fluid, or the path taken to reach the point, which is why two vessels of very different size can carry identical pressure at the same depth. The word vertical is doing real work in that sentence. On an inclined wall or a sloping pipe the distance measured along the surface is longer than the depth, and substituting it is the single most common way to get a hydrostatic question wrong. Layered fluids are handled by summing the contribution of each layer in turn, working downward from the surface. Watch the difference between gage and absolute pressure as well: gage pressure is measured relative to the atmosphere and is what the formula returns directly, while absolute pressure adds atmospheric pressure on top. A question that supplies an atmospheric value is usually asking for the absolute answer.

Key formulas

Gage pressure at depth h below a free surface: p = gamma * h
Specific weight from density: gamma = rho * g
Absolute from gage pressure: p_abs = p_atm + p_gage
Layered fluids, summed downward: p = sum(gamma_i * h_i)
Resultant force on a submerged plane surface: F = p_centroid * A = gamma * h_c * A

Common mistakes

Using the slant distance along an inclined surface instead of the vertical depth.
Mixing gage and absolute pressure, usually by adding atmospheric pressure when the question asked for gage.
Carrying the wrong specific weight for water: 9.81 kN/m^3 in SI, 62.4 lb/ft^3 in USCS.
Assuming pressure depends on the volume or width of the reservoir rather than on depth alone.

4 worked practice problems

Commit to an answer before opening the solution. Each problem shows the governing model, every substitution, a unit check, and why each wrong choice is wrong.

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Background reading

Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.

Hydrostatic Pressure practice problems for the FE Civil exam

Key formulas

Gage pressure at depth h below a free surface: p = gamma * h
Specific weight from density: gamma = rho * g
Absolute from gage pressure: p_abs = p_atm + p_gage
Layered fluids, summed downward: p = sum(gamma_i * h_i)
Resultant force on a submerged plane surface: F = p_centroid * A = gamma * h_c * A

Common mistakes

Using the slant distance along an inclined surface instead of the vertical depth.
Mixing gage and absolute pressure, usually by adding atmospheric pressure when the question asked for gage.
Carrying the wrong specific weight for water: 9.81 kN/m^3 in SI, 62.4 lb/ft^3 in USCS.
Assuming pressure depends on the volume or width of the reservoir rather than on depth alone.

4 worked practice problems

Commit to an answer before opening the solution. Each problem shows the governing model, every substitution, a unit check, and why each wrong choice is wrong.

Keep going in Fluid Mechanics

All Fluid Mechanics practice problems