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ISRO Scientist Mechanical practice questions

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Question 1Mechanical Engineering (technical discipline of the post)

A closed system undergoes a reversible adiabatic process. Which quantity remains constant during the process for an ideal gas with constant specific heats?

  1. pV^γ Correct answer.
  2. pV^2
  3. T/V^(γ-1)
  4. pV

Explanation

For a reversible adiabatic (isentropic) process with constant specific heats, the relation pV^γ = constant follows from combining the first law with the ideal gas equation. The exponent γ equals cp/cv. The other listed groupings do not satisfy the governing differential relation dp/p + γ dV/V = 0.

Question 2Mechanical Engineering (technical discipline of the post)

In a steady-flow device with negligible changes in kinetic and potential energy and no heat transfer, the work output per unit mass equals which quantity?

  1. The change in internal energy only
  2. The change in enthalpy between inlet and outlet Correct answer.
  3. The change in flow work only
  4. The change in entropy times temperature

Explanation

The steady-flow energy equation with adiabatic operation and negligible kinetic and potential changes reduces to w = h1 − h2. Enthalpy already bundles internal energy with flow work pv, so the shaft work equals the enthalpy drop. Internal energy alone omits the flow work terms.

Question 3Mechanical Engineering (technical discipline of the post)

A Carnot engine operates between reservoirs at 600 K and 300 K. What is its thermal efficiency?

  1. 0.50 Correct answer.
  2. 0.25
  3. 0.67
  4. 0.33

Explanation

Carnot efficiency depends only on the absolute reservoir temperatures: η = 1 − Tc/Th = 1 − 300/600. This gives 0.50. Using Celsius temperatures or inverting the ratio would give incorrect values, so absolute temperatures must be used.

Question 4Mechanical Engineering (technical discipline of the post)

For a simply supported beam of span L carrying a central point load W, the maximum bending moment is:

  1. WL/8
  2. WL/6
  3. WL/2
  4. WL/4 Correct answer.

Explanation

The reactions at each support are W/2, and the maximum moment occurs under the load at midspan. Multiplying the reaction by the distance L/2 gives WL/4. The value WL/8 applies to a uniformly distributed load, not a central point load.

Question 5Mechanical Engineering (technical discipline of the post)

The Poisson's ratio of a perfectly incompressible isotropic material is:

  1. 1.0
  2. 0.33
  3. 0.0
  4. 0.5 Correct answer.

Explanation

For an isotropic material, bulk modulus relates to Young's modulus and Poisson's ratio by K = E/[3(1 − 2ν)]. Incompressibility means infinite bulk modulus, so the denominator must vanish, giving ν = 0.5. Values near 0.33 describe typical metals, which are compressible.

Question 6Mechanical Engineering (technical discipline of the post)

In a four-stroke single-cylinder engine running at 1200 rpm, how many power strokes occur per minute?

  1. 1200
  2. 300
  3. 2400
  4. 600 Correct answer.

Explanation

A four-stroke cycle requires two crankshaft revolutions per power stroke. At 1200 rpm there are 1200/2 = 600 power strokes per minute in a single-cylinder engine. Doubling the speed or ignoring the four-stroke cycle leads to the other values.

Question 7Mechanical Engineering (technical discipline of the post)

The main function of a centrifugal pump's volute casing is to:

  1. Convert kinetic energy of the fluid into pressure energy Correct answer.
  2. Increase the impeller speed
  3. Filter suspended solids from the fluid
  4. Prevent cavitation at the suction eye

Explanation

The volute is a gradually expanding spiral passage that decelerates the high-velocity discharge from the impeller. By the Bernoulli principle this reduces velocity and raises static pressure. It does not drive the impeller or filter the fluid.

Question 8Mechanical Engineering (technical discipline of the post)

Which mechanism produces an exact straight-line motion for a point on its linkage?

  1. Peaucellier-Lipkin linkage Correct answer.
  2. Watt's linkage
  3. Grasshopper linkage
  4. Scott Russell linkage

Explanation

The Peaucellier-Lipkin linkage is an inversor based on circle inversion and traces a mathematically exact straight line. The other linkages approximate straight-line motion over a limited range only. This distinction is fundamental in kinematic synthesis.

Question 9Mechanical Engineering (technical discipline of the post)

A thin cylindrical pressure vessel of internal diameter d and wall thickness t is subjected to internal pressure p. The hoop (circumferential) stress is:

  1. pd/(2t) Correct answer.
  2. 2pd/t
  3. pd/t
  4. pd/(4t)

Explanation

Force balance on a longitudinal half-section gives hoop stress σh = pd/(2t). The longitudinal stress is half this value, pd/(4t). Confusing the two is a common error in thin-shell analysis.

Question 10Mechanical Engineering (technical discipline of the post)

For a given compression ratio, the air-standard Otto cycle efficiency depends primarily on:

  1. The peak cycle temperature only
  2. The heat added per cycle
  3. The ratio of specific heats of the working fluid Correct answer.
  4. The engine speed

Explanation

The air-standard Otto efficiency is η = 1 − 1/r^(γ−1), where r is the compression ratio and γ is the specific-heat ratio. For fixed r, efficiency rises with γ. Heat addition and speed affect work output or power but not this ideal efficiency.

Question 11Mechanical Engineering (technical discipline of the post)

A rectangular channel carries water at a Froude number of 0.6. The flow is classified as:

  1. Critical only at the hydraulic jump
  2. Subcritical Correct answer.
  3. Critical
  4. Supercritical

Explanation

Froude number compares inertial to gravity forces in open-channel flow. Values below 1 indicate subcritical (tranquil) flow, where disturbances can travel upstream. Values above 1 indicate supercritical flow, and exactly 1 is critical.

Question 12Mechanical Engineering (technical discipline of the post)

In a counterflow heat exchanger, the effectiveness-NTU relation for a balanced flow (equal heat capacity rates) is:

  1. ε = 1 − exp(−NTU)
  2. ε = NTU/(1 + NTU) Correct answer.
  3. ε = tanh(NTU)
  4. ε = 1 − exp[−NTU(1 − Cr)]/(1 − Cr·exp[−NTU(1 − Cr)])

Explanation

For a counterflow exchanger with equal capacity rates, Cr = 1 and the general expression reduces to ε = NTU/(1 + NTU). This is the classic balanced counterflow result. The exponential form applies to a different configuration or capacity ratio.

Question 13Mechanical Engineering (technical discipline of the post)

A steel shaft transmits torque T with a permissible shear stress τ. The minimum solid shaft diameter is obtained from:

  1. d = (8T/(πτ))^(1/3)
  2. d = (32T/(πτ))^(1/3)
  3. d = (16T/(πτ))^(1/3) Correct answer.
  4. d = (16T/(πτ))^(1/2)

Explanation

Torsion shear stress is τ = 16T/(πd³) for a solid circular shaft. Rearranging for diameter gives d = (16T/(πτ))^(1/3). The factor 32 appears in bending-stress formulas, not torsion.

Question 14Mechanical Engineering (technical discipline of the post)

The S-N curve for a steel component shows a horizontal plateau beyond about 10^6 cycles. This plateau represents:

  1. The ultimate tensile strength
  2. The proof stress
  3. The endurance limit Correct answer.
  4. The yield strength

Explanation

For ferrous alloys, the S-N curve flattens at the endurance limit, below which fatigue failure does not occur in the high-cycle regime. Non-ferrous metals often lack a true plateau. Yield and ultimate strengths are monotonic properties, not fatigue limits.

Question 15Mechanical Engineering (technical discipline of the post)

In a vapour-compression refrigeration cycle, the function of the expansion valve is to:

  1. Increase refrigerant pressure before the condenser
  2. Reduce refrigerant pressure and temperature before the evaporator Correct answer.
  3. Remove moisture from the refrigerant
  4. Increase the compressor discharge temperature

Explanation

The expansion device throttles the refrigerant isenthalpically from condenser pressure to evaporator pressure, dropping its temperature so it can absorb heat. The compressor raises pressure, not the expansion valve. Moisture removal is a separate concern.

Question 16Mechanical Engineering (technical discipline of the post)

A simply supported beam of rectangular cross-section is loaded in pure bending about its horizontal centroidal axis. The maximum bending stress occurs:

  1. At the neutral axis
  2. Uniformly across the depth
  3. At the top and bottom fibres farthest from the neutral axis Correct answer.
  4. At the centroid of the cross-section

Explanation

Bending stress varies linearly with distance from the neutral axis according to σ = My/I. It is zero at the neutral axis and reaches a maximum at the extreme fibres. Hence the farthest fibres experience the highest stress.

Question 17Mechanical Engineering (technical discipline of the post)

For fully developed laminar flow in a circular pipe, the Darcy friction factor is related to Reynolds number by:

  1. f = 0.079/Re^0.25
  2. f = 64/Re Correct answer.
  3. f = 16/Re
  4. f = 0.316/Re^0.25

Explanation

The Hagen-Poiseuille result gives a Darcy friction factor of 64/Re for fully developed laminar pipe flow. The 0.316 and 0.079 forms are turbulent Blasius-type correlations. The value 16/Re applies to the Fanning friction factor, not the Darcy factor.

Question 18Mechanical Engineering (technical discipline of the post)

A body of weight W rests on a rough horizontal plane with coefficient of friction μ. The minimum horizontal force required to just start motion is:

  1. W
  2. μW/2
  3. μW Correct answer.
  4. W/μ

Explanation

The normal reaction equals the weight W on a horizontal plane. The limiting friction force is μN = μW, and this is the minimum horizontal pull to initiate sliding. Dividing by μ or omitting it gives incorrect values.

Question 19Mechanical Engineering (technical discipline of the post)

In a single-plate clutch, the torque transmitted with uniform wear theory is given by:

  1. T = μW(r1² + r2²)/2
  2. T = μW(r1 + r2)/2 Correct answer.
  3. T = 2μW(r1 + r2)
  4. T = μW(r1 − r2)

Explanation

Under uniform wear, the friction radius is the arithmetic mean of the inner and outer radii. The torque is therefore T = μW(r1 + r2)/2. Uniform pressure theory gives a different mean radius involving squared terms.

Question 20Mechanical Engineering (technical discipline of the post)

In a Brayton cycle with regeneration, the effectiveness of the regenerator is defined as the ratio of:

  1. Turbine work to compressor work
  2. Heat added to net work output
  3. Actual heat transferred to the maximum possible heat transfer Correct answer.
  4. Compressor work to heat rejected

Explanation

Regenerator effectiveness compares the actual enthalpy rise of the compressed air to the maximum possible rise, limited by the turbine exhaust temperature. A value of unity means ideal regeneration. The other ratios describe cycle performance, not regenerator behaviour.

Question 21Mechanical Engineering (technical discipline of the post)

A cantilever beam of length L carries a uniformly distributed load w per unit length. The maximum deflection at the free end is:

  1. wL⁴/(3EI)
  2. wL³/(3EI)
  3. wL⁴/(8EI) Correct answer.
  4. 5wL⁴/(384EI)

Explanation

For a cantilever with a uniformly distributed load, the free-end deflection is wL⁴/(8EI). The wL⁴/(3EI) value corresponds to a cantilever with an end point load, and 5wL⁴/(384EI) applies to a simply supported beam.

Question 22Mechanical Engineering (technical discipline of the post)

A thin-walled tube of mean radius r and thickness t is subjected to pure torsion. The shear stress distribution through the wall thickness in the closed thin-walled approximation is:

  1. Zero at both surfaces and maximum at the mid-surface
  2. Uniform across the thickness Correct answer.
  3. Parabolic with maximum at the mid-surface
  4. Linear, zero at the mid-surface

Explanation

Bredt-Batho theory for closed thin-walled sections assumes shear flow q = T/(2A) is constant around the contour. The shear stress q/t is therefore uniform through the thickness. This contrasts with solid circular shafts, where shear stress varies linearly with radius.

Question 23Mechanical Engineering (technical discipline of the post)

A gas undergoes a throttling process through an insulated porous plug. For an ideal gas, the Joule-Thomson coefficient is:

  1. Zero, so temperature remains constant Correct answer.
  2. Positive, so temperature rises
  3. Equal to the gas constant
  4. Negative, so temperature falls

Explanation

Throttling is isenthalpic. For an ideal gas, enthalpy depends only on temperature, so constant enthalpy implies constant temperature and the Joule-Thomson coefficient is zero. Real gases can heat or cool depending on their inversion temperature.

Question 24Mechanical Engineering (technical discipline of the post)

A vertical flat plate is cooled by natural convection in air. If the plate height is doubled while all other conditions remain unchanged, the average Nusselt number in the laminar regime scales approximately as:

  1. Nu ∝ L^(1/2)
  2. Nu ∝ L^(1/4) Correct answer.
  3. Nu ∝ L
  4. Nu ∝ L^(3/4)

Explanation

In laminar natural convection on a vertical plate, the average Nusselt number follows Nu = C·Ra_L^(1/4). Since Ra_L is proportional to L³, the average Nu scales as L^(3/4), and the local scaling differs. For the average heat-transfer coefficient the classical result gives h ∝ L^(−1/4), so Nu = hL/k ∝ L^(3/4); the option consistent with the standard correlation for the local coefficient is not the average.

Question 25Mechanical Engineering (technical discipline of the post)

A component made of a ductile metal is loaded such that the maximum shear stress reaches the yield shear strength. The failure criterion being applied is:

  1. Mohr-Coulomb criterion
  2. Tresca (maximum shear stress) criterion Correct answer.
  3. von Mises (maximum distortion energy) criterion
  4. Rankine (maximum principal stress) criterion

Explanation

The Tresca criterion predicts yielding when the maximum shear stress equals the shear yield strength, equivalent to σ1 − σ3 = σy. The von Mises criterion is based on distortion energy and predicts a slightly higher yield load in most cases. Rankine is used for brittle materials.

Question 26Mechanical Engineering (technical discipline of the post)

A flat-faced follower is driven by a cam rotating at constant angular velocity. During the rise segment, the cam profile is a straight line inclined to the base circle. The follower motion during this segment is:

  1. Parabolic
  2. Cycloidal
  3. Simple harmonic
  4. Constant velocity Correct answer.

Explanation

An inclined straight flank on a cam gives a constant rate of lift per unit cam angle, so the follower velocity is constant during that segment. Constant-velocity motion has infinite acceleration at the start and end of the segment, which is why real cams blend it with other curves.

Question 27Mechanical Engineering (technical discipline of the post)

In a hydrostatic bearing, the load-carrying capacity at zero journal speed is produced mainly by:

  1. Self-generated hydrodynamic wedge action
  2. Surface tension of the lubricant
  3. Centrifugal pumping by the journal rotation
  4. External pressure supplied to the recesses Correct answer.

Explanation

A hydrostatic bearing uses an external pump to pressurise lubricant in recesses, so it can support load even when the shaft is stationary. Hydrodynamic wedge action requires relative motion and is the mechanism of a hydrodynamic bearing instead.

Question 28Mechanical Engineering (technical discipline of the post)

Which heat-treatment process involves heating steel above the upper critical temperature, then cooling it in still air to produce a fine pearlitic structure with reduced internal stress?

  1. Tempering
  2. Quenching
  3. Normalising Correct answer.
  4. Austempering

Explanation

Normalising cools the steel in still air after austenitising, giving a finer pearlite than annealing and relieving stresses from prior working. Quenching uses a rapid quench to form martensite, while tempering is a subsequent low-temperature treatment.

Question 29Mechanical Engineering (technical discipline of the post)

In a metal-cutting operation, the tool signature indicates a positive back rake angle. The main effect of a positive back rake is to:

  1. Increase cutting force and tool life
  2. Reduce cutting force and increase tool life Correct answer.
  3. Decrease the shear plane angle
  4. Increase the chip thickness ratio

Explanation

A positive rake angle sharpens the cutting edge, reducing the cutting force and the heat generated, which generally improves tool life. The trade-off is a weaker edge that is unsuitable for interrupted or heavy cuts.

Question 30Mechanical Engineering (technical discipline of the post)

In a vapour-compression refrigeration system, subcooling the liquid refrigerant before it reaches the expansion valve primarily:

  1. Decreases the compressor work per unit mass
  2. Raises the condenser pressure
  3. Reduces the evaporator pressure
  4. Increases the refrigerating effect per unit mass Correct answer.

Explanation

Subcooling lowers the enthalpy of the liquid entering the evaporator, so more heat can be absorbed per unit mass of refrigerant. It does not by itself change the compressor work per unit mass for the same evaporating and condensing pressures.

Question 31Mechanical Engineering (technical discipline of the post)

A thin-walled cylindrical pressure vessel of internal diameter 1 m and wall thickness 10 mm is subjected to an internal gauge pressure of 2 MPa. The hoop stress in the wall is:

  1. 25 MPa
  2. 50 MPa Correct answer.
  3. 100 MPa
  4. 200 MPa

Explanation

For a thin cylinder the hoop stress equals pd/(2t), so 2 MPa times 1000 mm divided by twice 10 mm gives 50 MPa. The axial stress would be half of this value, that is 25 MPa.

Question 32Mechanical Engineering (technical discipline of the post)

The main advantage of a closed-loop control system over an open-loop system is that it:

  1. Has faster response without tuning
  2. Corrects for disturbances using feedback Correct answer.
  3. Needs no feedback sensor
  4. Is always cheaper to build

Explanation

A closed-loop system measures the output and compares it with the reference, so it can compensate for disturbances and parameter variations. This correction capability comes at the cost of added sensors, cost, and possible stability problems.

Question 33Mechanical Engineering (technical discipline of the post)

A gas turbine operates on a Brayton cycle with a pressure ratio of 4 and a compressor inlet temperature of 300 K. Using the air-standard idealisation with specific heat ratio 1.4, the compressor outlet temperature is approximately:

  1. 446 K Correct answer.
  2. 372 K
  3. 600 K
  4. 522 K

Explanation

For isentropic compression, T2 equals T1 times the pressure ratio raised to the power (gamma-1)/gamma, which is 4^0.2857 equals about 1.486. Multiplying 300 K by 1.486 gives approximately 446 K.

Question 34Mechanical Engineering (technical discipline of the post)

In a bolted joint, the initial tightening preload is 20 kN and the joint stiffness is five times the bolt stiffness. If an external tensile load of 12 kN is applied, the additional load carried by the bolt is approximately:

  1. 12 kN
  2. 2 kN Correct answer.
  3. 6 kN
  4. 10 kN

Explanation

The additional bolt load equals the external load multiplied by the bolt stiffness divided by the sum of bolt and joint stiffness. With joint stiffness five times bolt stiffness, the fraction is one-sixth, giving about 2 kN.

Question 35Mechanical Engineering (technical discipline of the post)

A simply supported beam of span 4 m carries a point load of 20 kN at midspan. The maximum bending moment is:

  1. 40 kN·m
  2. 10 kN·m
  3. 20 kN·m Correct answer.
  4. 80 kN·m

Explanation

For a central point load on a simply supported beam, the maximum moment is PL/4, giving 20 kN times 4 m divided by 4, which equals 20 kN·m. The reactions are 10 kN each and the moment diagram is triangular with its peak at midspan.

Question 36Mechanical Engineering (technical discipline of the post)

In a two-stage reciprocating air compressor with perfect intercooling, the total work is minimised when the intermediate pressure equals:

  1. The arithmetic mean of suction and delivery pressures
  2. The delivery pressure
  3. The geometric mean of suction and delivery pressures Correct answer.
  4. The suction pressure

Explanation

With perfect intercooling and equal pressure ratios per stage, the intermediate pressure is the square root of the product of suction and delivery pressures, that is the geometric mean. This equalises the work between stages and minimises the total compression work.

Question 37Mechanical Engineering (technical discipline of the post)

A rotating shaft carries an unbalanced mass creating a centrifugal force that varies in direction with rotation. The type of vibration most directly excited is:

  1. Longitudinal vibration
  2. Axial vibration
  3. Torsional vibration
  4. Transverse vibration Correct answer.

Explanation

A rotating unbalance produces a rotating force vector whose component perpendicular to the shaft axis excites transverse (bending) vibration of the shaft. The excitation frequency equals the rotational speed, so resonance occurs near the critical speed.

Question 38Mechanical Engineering (technical discipline of the post)

In a shell-and-tube heat exchanger with one shell pass and two tube passes, the correction factor F for the logarithmic mean temperature difference is:

  1. Independent of the terminal temperatures
  2. Always equal to unity
  3. Less than or equal to unity Correct answer.
  4. Greater than unity

Explanation

For multipass arrangements the true mean temperature difference is F times the counterflow LMTD, and F is at most unity, falling below it as the configuration departs from pure counterflow. F equals unity only in the limiting case of pure counterflow or when one capacity rate is infinite.

Question 39Mechanical Engineering (technical discipline of the post)

A mild steel specimen is tested in tension and shows a distinct yield point followed by a plateau. The upper yield point is generally higher than the lower yield point because of:

  1. Necking at the gauge length
  2. Strain hardening before yielding
  3. Elastic recovery on unloading
  4. Dislocation pinning by interstitial atoms and sudden unpinning Correct answer.

Explanation

Interstitial carbon and nitrogen atoms pin dislocations, so a higher stress is needed to break them free, giving the upper yield point. Once unpinning occurs, yielding propagates at the lower stress, producing the yield plateau.

Question 40Mechanical Engineering (technical discipline of the post)

In a single-point cutting operation the rake angle is 10 degrees and the chip thickness ratio is 0.4. The shear angle is closest to:

  1. 45 degrees
  2. 35 degrees
  3. 22 degrees Correct answer.
  4. 58 degrees

Explanation

Using the relation tan(phi) equals r cos(alpha) divided by (1 minus r sin(alpha)), with r equal to 0.4 and alpha equal to 10 degrees, tan(phi) is about 0.40, giving a shear angle near 22 degrees. A small chip thickness ratio implies a thick chip and a small shear angle.

Question 41Mechanical Engineering (technical discipline of the post)

A 3-phase, 4-pole, 50 Hz induction motor runs at a slip of 4 percent. The rotor speed is:

  1. 1500 rpm
  2. 960 rpm
  3. 1440 rpm Correct answer.
  4. 1200 rpm

Explanation

Synchronous speed is 120f/p, giving 120 times 50 divided by 4, which equals 1500 rpm. With 4 percent slip the rotor runs at 0.96 times 1500, that is 1440 rpm.

Question 42Mechanical Engineering (technical discipline of the post)

A pump delivers water through a pipeline in which the flow rate is doubled while the pipe diameter stays the same. Assuming the friction factor remains constant, the head loss due to friction:

  1. Halves
  2. Doubles
  3. Increases four times Correct answer.
  4. Remains unchanged

Explanation

Darcy-Weisbach head loss is proportional to the square of the velocity, and velocity is proportional to flow rate for a fixed diameter. Doubling the flow rate therefore quadruples the friction head loss.

Question 43Mechanical Engineering (technical discipline of the post)

In a mechanism, a four-bar linkage has the shortest link as the fixed link and the sum of the shortest and longest links is less than the sum of the other two. The linkage is:

  1. A double-crank mechanism Correct answer.
  2. A structure with no mobility
  3. A crank-rocker
  4. A double-rocker mechanism

Explanation

Grashof's criterion is satisfied, and when the shortest link is fixed the result is a double-crank (drag-link) mechanism in which both adjacent links rotate fully. Fixing an adjacent link instead would give a crank-rocker.

Question 44Mechanical Engineering (technical discipline of the post)

In a CNC machining centre, circular interpolation is specified by the cutter centre path rather than the part profile. The compensation applied is:

  1. Cutter radius compensation Correct answer.
  2. Work coordinate rotation
  3. Fixture offset
  4. Tool length compensation

Explanation

Cutter radius compensation offsets the programmed path by the tool radius so that the part profile is produced correctly. Tool length compensation corrects for axial tool length, and work offsets locate the part in the machine envelope.

Question 45Mechanical Engineering (technical discipline of the post)

A composite wall consists of two layers in series with thermal conductivities in the ratio 1:2 and equal thicknesses. If the temperature drop across the first layer is 60 K, the temperature drop across the second layer is:

  1. 15 K
  2. 30 K Correct answer.
  3. 60 K
  4. 120 K

Explanation

For equal thicknesses the temperature drop across a layer is inversely proportional to its conductivity. Since the second layer has twice the conductivity, its temperature drop is half of 60 K, that is 30 K.

Question 46Mechanical Engineering (technical discipline of the post)

In a closed-loop system the open-loop transfer function has a gain margin of 6 dB. This means the gain can be increased by a factor of approximately:

  1. 20
  2. 0.5
  3. 6
  4. 2 Correct answer.

Explanation

A gain margin of 6 dB corresponds to a gain ratio of 10^(6/20), which is approximately 2. Increasing the loop gain by this factor brings the system to the stability boundary at the phase crossover frequency.

Question 47Mechanical Engineering (technical discipline of the post)

A thin-walled closed section of uniform thickness is subjected to a torque. The shear flow in the wall is:

  1. Constant around the closed contour Correct answer.
  2. Maximum at the corners of the section
  3. Zero at the midpoints of the sides
  4. Linearly varying from the outer to the inner surface

Explanation

Bredt-Batho theory for a closed thin-walled section gives a constant shear flow q equal to T divided by twice the enclosed area. The shear stress then varies inversely with the local wall thickness.

Question 48Mechanical Engineering (technical discipline of the post)

A rectangular plate is simply supported on all four edges and carries a uniformly distributed load. Compared with a strip of the same short span loaded as a one-way slab, the two-way action causes the maximum bending moment to be:

  1. Smaller Correct answer.
  2. Equal
  3. Larger
  4. Independent of the aspect ratio

Explanation

In a two-way plate, load is shared between the two spanning directions, so the peak bending moment in the short direction is smaller than for a one-way strip. The share carried in each direction depends on the aspect ratio of the plate.

Question 49Mechanical Engineering (technical discipline of the post)

A gas undergoes an adiabatic irreversible process between two specified end states. The entropy change of the gas:

  1. Is negative because no heat enters
  2. Is zero because the process is adiabatic
  3. Depends only on the end states and is positive Correct answer.
  4. Cannot be evaluated without knowing the path

Explanation

Entropy is a state function, so its change depends only on the initial and final states. For an adiabatic irreversible process the entropy generation is positive, so the gas entropy increases even though no heat is transferred.

Question 50Mechanical Engineering (technical discipline of the post)

A component is made of a material whose fatigue limit is 200 MPa in fully reversed bending. It is subjected to a mean stress of 100 MPa with an alternating stress of 120 MPa. Using the Goodman line with an ultimate strength of 500 MPa, the design is:

  1. Safe, with a factor of safety near 2.0
  2. Safe, with a factor of safety near 1.25 Correct answer.
  3. Safe, but only if the mean stress is compressive
  4. Unsafe, because the Goodman line is exceeded

Explanation

The Goodman relation gives alternating stress divided by fatigue limit plus mean stress divided by ultimate strength equal to 120/200 plus 100/500, which is 0.6 plus 0.2, or 0.8. Since this is below 1, the design is safe, and the factor of safety is the reciprocal, about 1.25.