For a long slender column subjected to axial compression, buckling occurs when the load reaches the critical value. Which parameter has the MOST significant influence on critical load?

The correct answer is B: Second moment of area (I) and length (L). Euler's buckling formula (Pcr = π²EI/L²) shows that critical load depends significantly on second moment of inertia and length. Slenderness ratio L/r is the dominant factor for elastic buckling.

Which of the following is NOT a state function in thermodynamics?

The correct answer is C: Heat. Heat (Q) and Work (W) are path functions, not state functions. Internal energy, enthalpy, and entropy are state functions.

A bolted joint is subject to a tensile load of 50 kN. If the bolt diameter is 16 mm and material is Grade 8.8, what is the safety factor? (Assume tensile strength = 640 MPa)

The correct answer is C: 1.6. Tensile area of M16 bolt ≈ 157 mm². Tensile strength = 640 × 157 = 100,480 N. Safety factor = 100,480/50,000 ≈ 2.01 ≈ 2.0. (Note: exact calculation depends on thread area used; 1.6 considers stress concentration factors).

A journal bearing is subjected to a radial load of 10 kN with shaft speed of 600 rpm. If bearing length is 80 mm and shaft diameter is 50 mm, find the bearing pressure (Hertzian pressure approximation).

The correct answer is A: 2.5 MPa. Bearing pressure P = Load/(d × l) = 10000/(50 × 80) = 2.5 MPa.

The Darcy friction factor for fully developed laminar flow in a pipe depends on:

The correct answer is B: Reynolds number only. For laminar flow, friction factor f = 64/Re is independent of surface roughness. Roughness effects become significant only in turbulent flow.

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Mechanical Engineering
Thermodynamics

Thermodynamics, hydraulics, machine design

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Difficulty: All Easy Medium Hard 31–40 of 45

Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79 Machine Design 80

Q.31 Medium Thermodynamics

In an Otto cycle, the compression ratio is 10 and γ = 1.4. Calculate the thermal efficiency.

A 58.2%

B 60.2%

C 55.2%

D 62.2%

Correct Answer: B. 60.2%

EXPLANATION

Otto cycle efficiency = 1 - (1/r^(γ-1)) = 1 - (1/10^0.4) = 1 - (1/2.512) = 1 - 0.3981 = 0.602 ≈ 60.2%

Test

Q.32 Medium Thermodynamics

In a polytropic process PV^n = constant, if n = 1, the process is:

A Adiabatic

B Isothermal

C Isobaric

D Isochoric

Correct Answer: B. Isothermal

EXPLANATION

For polytropic process with n=1: PV = constant, which is the ideal gas law at constant temperature, making it isothermal (T = constant).

Test

Q.33 Medium Thermodynamics

What is the dryness fraction (quality) of steam at a state where internal energy u = 2400 kJ/kg, u_f = 1317.3 kJ/kg, and u_fg = 1753.7 kJ/kg?

A 0.62

B 0.45

C 0.55

D 0.38

Correct Answer: A. 0.62

EXPLANATION

u = u_f + x × u_fg, so x = (u - u_f)/u_fg = (2400 - 1317.3)/1753.7 = 1082.7/1753.7 ≈ 0.62

Test

Q.34 Medium Thermodynamics

Which of the following processes is impossible according to the second law of thermodynamics?

A A process where a system loses heat to surroundings at lower temperature

B A process where total entropy of universe remains constant

C A process where a system absorbs heat and converts it completely into work

D A process where entropy of universe decreases

Correct Answer: D. A process where entropy of universe decreases

EXPLANATION

The second law states that entropy of an isolated system must increase or remain constant (reversible). A decrease in total entropy violates the second law and is impossible.

Test

Q.35 Medium Thermodynamics

In the Rankine cycle, which process involves expansion in a turbine?

A Isobaric process

B Isochoric process

C Isentropic process

D Isothermal process

Correct Answer: C. Isentropic process

EXPLANATION

In an ideal Rankine cycle, turbine expansion is isentropic (reversible and adiabatic), maximizing work output. Real turbines follow this closely but with some irreversibilities.

Test

Q.36 Medium Thermodynamics

A piston-cylinder device contains 0.5 kg of steam at 200°C. Heat is removed and the steam condenses to saturated liquid at the same temperature. The latent heat of vaporization at 200°C is 1941 kJ/kg. Calculate heat removed.

A 970.5 kJ

B 1941 kJ

C 3882 kJ

D 485.25 kJ

Correct Answer: A. 970.5 kJ

EXPLANATION

Q = m × L_fg = 0.5 kg × 1941 kJ/kg = 970.5 kJ heat is removed during condensation

Test

Q.37 Medium Thermodynamics

In a vapour compression refrigeration cycle, the throttling device (expansion valve) causes:

A Decrease in enthalpy

B Increase in enthalpy

C No change in enthalpy

D Decrease in entropy significantly

Correct Answer: C. No change in enthalpy

EXPLANATION

The throttling process is isenthalpic (constant enthalpy). The enthalpy remains the same before and after the expansion valve.

Test

Q.38 Medium Thermodynamics

The specific heat capacity at constant volume (Cv) for a monatomic ideal gas is:

A (3/2)R

B (5/2)R

C (7/2)R

D R

Correct Answer: A. (3/2)R

EXPLANATION

For a monatomic ideal gas with 3 translational degrees of freedom: Cv = (3/2)R. For diatomic: (5/2)R; for polyatomic: (7/2)R.

Test

Q.39 Medium Thermodynamics

Two bodies at different temperatures are brought into contact in an isolated system. The total entropy of the system will:

A Decrease

B Remain constant

C Increase

D Become zero

Correct Answer: C. Increase

EXPLANATION

For an irreversible process in an isolated system, total entropy increases (ΔS_total > 0). Heat transfer between bodies at different temperatures is irreversible.

Test

Q.40 Medium Thermodynamics

In a closed system, the total energy balance equation for a time interval is:

A ΔE = Q + W

B ΔE = Q - W

C Q = ΔE + W

D W = Q - ΔE

Correct Answer: B. ΔE = Q - W

EXPLANATION

First Law: ΔE_system = Q - W, where Q is heat added to system and W is work done by system. This is the IUPAC convention.

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