Motor Vehicle Inspector / Assistant Motor Vehicle Inspector — 2023 — Official Paper — Kerala PSC PYQ Practice with Answers

Correct answer (Option A):\nIn liquids, the primary cause of viscosity is the cohesive forces between molecules. As the temperature rises, the thermal energy of the molecules increases, causing them to move further apart and weakening these attractive inter-molecular cohesive forces. Consequently, the resistance to flow decreases, leading to a reduction in liquid viscosity.\n\nWhy others are wrong:\nOption B is incorrect because cohesive forces decrease, not increase, with temperature. Options C and D are incorrect because molecular momentum transfer is the dominating factor for viscosity in gases, where viscosity increases with temperature due to increased momentum transfer, rather than in liquids.\n\nStudy tip:\nRemember the contrasting behavior of fluid viscosity with temperature: liquid viscosity decreases with temperature (dominated by cohesive forces), whereas gas viscosity increases with temperature (dominated by molecular momentum transfer).

Correct answer (Option C):\nAccording to Newton's law of viscosity, the shear stress (τ) acting on a fluid layer is directly proportional to the rate of shear strain or velocity gradient, which represents the deformation rate. \n\nFormula: τ = μ × (du/dy)\nWhere μ is the dynamic viscosity and du/dy is the deformation rate. Thus, shear stress changes linearly with the rate of deformation.\n\nWhy others are wrong:\nOption A and B are incorrect because viscosity (μ) acts as the constant of proportionality in the linear equation rather than being a variable to which shear stress is simply proportional or inversely proportional. Option D is incorrect because shear stress is proportional to the rate of shear strain, not the absolute shear strain itself (the latter applies to elastic solids under Hooke's law).\n\nStudy tip:\nAlways distinguish between solids and fluids: solids experience stress proportional to strain, while Newtonian fluids experience stress proportional to the rate of strain.

Correct answer (Option A):\nThe flash point is the lowest temperature at which a liquid fuel gives off sufficient vapor to form an ignitable mixture with air, producing a momentary flash when an ignition source is applied. The fire point is the temperature at which the fuel vapors continue to burn for at least 5 seconds after ignition. Because sustained combustion requires a higher vapor production rate, additional heating is necessary, making the fire point always higher than the flash point (typically by about 5°C to 30°C).\n\nWhy others are wrong:\nOption B is fundamentally wrong because the fire point cannot be lower than the flash point. Option C is wrong because they are closely related thermodynamic safety characteristics of fuel volatile properties. Option D is incorrect as the hierarchy is fixed; the fire point never drops below the flash point for a given fuel.\n\nStudy tip:\nThink of the flash point as a temporary ignitable state, whereas the fire point is a continuous combustion state requiring higher thermal energy.

Correct answer (Option A):\nIn a centrifugal pump, the fluid is drawn into the center of the rotating impeller, a region known as the 'eye' of the pump. As the impeller rotates, it imparts kinetic energy to the fluid, forcing it radially outward by centrifugal action into the volute casing where kinetic energy transforms into pressure energy.\n\nWhy others are wrong:\nOptions B, C, and D are incorrect description of the operational design of standard centrifugal impellers. The liquid does not enter initially from the outer periphery, top, bottom, or sides; it must enter axially at the center to be slung outward efficiently.\n\nStudy tip:\nVisualize the fluid flow path in a centrifugal pump: axial entry at the central impeller eye, followed by radial acceleration outward into the discharge casing.

Correct answer (Option A):\nUniform flow is defined as a type of fluid flow in which the flow parameters, such as velocity, do not change with respect to space coordinates (position) at any specific instant of time. Mathematically, it is expressed as ∂V/∂s = 0, where t is constant.\n\nWhy others are wrong:\nOption B refers to flow where fluid density varies. Option C defines steady flow, which is where parameters do not change with respect to time at a specific location (∂V/∂t = 0). Option D refers to a flow where fluid elements experience rotation about their own axes.\n\nStudy tip:\nKeep the flow definitions clear: Space-independent velocity = Uniform Flow; Time-independent velocity = Steady Flow.