Mechanical Engineering Flashcards

On the NCEES PE Mechanical exam, the thermal resistance of a cylindrical pipe wall for radial conduction is:

Answer: R = ln(r₂/r₁)/(2πkL)

For a hollow cylinder, thermal resistance R = ln(r₂/r₁)/(2πkL), accounting for the changing area with radius.

In NCEES PE Mechanical exam problems, the second law of thermodynamics implies that:

Answer: Entropy of an isolated system can only stay the same or increase

The second law states that entropy of an isolated system never decreases; irreversibilities cause entropy generation.

In NCEES PE Mechanical design, which factor most directly determines the selection of a rolling element bearing for a given application?

Answer: Dynamic load rating C and required service life L10

Bearing selection is based on the dynamic load rating C and the L10 life (hours/cycles at 90% reliability) for the design load.

For an ideal gas in a polytropic process (PVⁿ = constant) on the NCEES PE exam, an isentropic process corresponds to n equal to:

Answer: γ (ratio of specific heats)

An isentropic (reversible adiabatic) process follows PVⁿ = constant where n = γ = Cp/Cv.

In NCEES PE Mechanical exam fluid mechanics, the continuity equation for steady incompressible flow states:

Answer: A₁V₁ = A₂V₂ (volumetric flow rate is constant)

For steady incompressible flow, conservation of mass gives A₁V₁ = A₂V₂ — the flow rate Q is conserved.

In NCEES Mechanical Engineering, which type of stress occurs in a thin-walled pressure vessel in the circumferential (hoop) direction?

Answer: Hoop (circumferential) tensile stress = pr/t

Hoop stress in a thin-walled cylinder σ_h = pr/t governs wall thickness design for internal pressure vessels.