Q60. When water is poured slowly from a jug and clings to its side, which property chiefly accounts for this behaviour?
[UPSC NDA-II – 2026]
(A) Surface tension
(B) Low vapour pressure
(C) Negligible compressibility
(D) Low viscosity
Answer: (A) Surface tension
Explanation: Surface tension, together with adhesion to the jug surface, makes water follow the spout or edge rather than separate immediately. Cohesive attraction helps maintain the thin liquid sheet. The phenomenon is familiar when water trickles down a vessel while pouring.
Q59. Which physical property is generally characteristic of metals?
[RRB Group D – August 2026]
(A) They conduct heat poorly
(B) They have metallic lustre and conduct electricity well
(C) They are invariably brittle
(D) Every metal is solid at room temperature
Answer: (B) They have metallic lustre and conduct electricity well
Explanation: Most metals reflect light strongly and contain mobile electrons, which help carry electrical current. They also tend to conduct heat efficiently. Mercury is liquid at ordinary room temperature, so a claim that every metal is solid would be incorrect.
Q58. Of sulphur, diamond and bromine, which property would be least likely to be common to all three samples?
[RRB Group D – August 2026]
(A) Poor thermal conduction
(B) High electrical conductivity
(C) Brittleness
(D) Dull appearance
Answer: (B) High electrical conductivity
Explanation: These are non-metallic substances, and high electrical conductivity is not their shared characteristic. Diamond conducts heat exceptionally well but is a poor electrical conductor; bromine is liquid at room temperature. Generalisations about other properties should therefore be treated cautiously.
Q57. How do non-metals generally behave in terms of electrical conduction in their pure forms?
[RRB Group D – August 2026]
(A) They are excellent conductors
(B) They usually do not conduct electricity
(C) They conduct as efficiently as metals
(D) They always have metallic conductivity
Answer: (B) They usually do not conduct electricity
Explanation: Most non-metals lack freely moving charge carriers, making them poor electrical conductors. Graphite is an important exception because its delocalised electrons can carry current. The examination tests the general trend, not an exception-free rule for every non-metal.
Q56. Consider ocean currents:
(1) water-density differences drive important currents;
(2) seawater density depends on temperature, salinity and pressure;
(3) currents always flow from denser to less dense water.
Which statements are correct?
[UPSC CDS-I – 2026]
(A) 1 and 2 only
(B) 1 only
(C) 1, 2 and 3
(D) 2 and 3 only
Answer: (A) 1 and 2 only
Explanation: Differences in temperature and salinity produce density contrasts that drive thermohaline circulation. Pressure also affects seawater density. The proposed universal flow direction is oversimplified: currents depend on pressure gradients, wind, gravity, Earth rotation and vertical density structure.
Q55. For a floating object in stable equilibrium, where must its centre of gravity lie?
[UPSC NDA-I – 2026]
(A) Above its centre of buoyancy
(B) Below its centre of buoyancy in every case
(C) Exactly at its centre of buoyancy
(D) Below its metacentre
Answer: (D) Below its metacentre
Explanation: A freely floating body is stable against small tilting when its metacentre lies above its centre of gravity. That position produces a restoring moment. The centre of gravity need not lie below the centre of buoyancy for stability.
Q54. What term refers to the gaps or spaces between constituent particles in a solid?
[RRB Group D – January 2026]
(A) Inter-particle space
(B) Density
(C) Rigidity
(D) Compressibility
Answer: (A) Inter-particle space
Explanation: Inter-particle spaces are the microscopic gaps between atoms, ions or molecules. They are usually comparatively small in solids, where particles occupy closely packed positions. Rigidity describes resistance to deformation, whereas density expresses the amount of mass per unit volume.
Q53. A hydrometer measures the density of a liquid. Which physical principle explains its operation?
[RRB Group D – December 2025]
(A) Pascal’s law
(B) Universal gravitation alone
(C) Archimedes’ principle
(D) Newton’s first law
Answer: (C) Archimedes’ principle
Explanation: A hydrometer floats at a depth determined by the liquid density. Archimedes’ principle states that buoyant force equals the weight of fluid displaced. The instrument sinks further in less-dense liquids and floats higher when liquid density increases.
Q52. Which is the SI unit used to express the density of matter?
[RRB Group D – December 2025]
(A) Kilogram per cubic metre
(B) Kilogram per square metre
(C) Kilogram per metre
(D) Gram per metre
Answer: (A) Kilogram per cubic metre
Explanation: Density is mass divided by volume. In the SI system, mass is measured in kilograms and volume in cubic metres, giving kg/m³. The unit describes the mass contained in each unit volume of a substance at specified conditions.
Q51. Which property normally describes solid non-metals?
[RRB Group D – December 2025]
(A) High density and high melting point without exception
(B) Malleability and ductility
(C) Brittleness
(D) Excellent electrical conductivity
Answer: (C) Brittleness
Explanation: Many solid non-metals fracture when force is applied instead of being rolled into sheets or drawn into wires. They are consequently described as brittle. Graphite and diamond illustrate how non-metals can nevertheless have markedly different physical properties.
Q50. Why are liquids far less compressible than gases?
[RRB Group D – December 2025]
(A) They always contain dissolved gases
(B) They are always less dense than solids
(C) Their mass cannot change
(D) Their particles have relatively little empty space between them
Answer: (D) Their particles have relatively little empty space between them
Explanation: Liquid particles are packed much closer than gas particles, leaving comparatively little space available for compression. As a result, applying pressure causes a relatively small volume change. Most liquids still have finite compressibility, though it is low.
Q49. Which typical property explains why many metals make a ringing sound when struck?
[RRB Group D – December 2025]
(A) Brittleness
(B) Plasticity
(C) Sonority
(D) Low conductivity
Answer: (C) Sonority
Explanation: Sonority means that a material can produce a ringing or resonant sound when struck. Metallic solids can vibrate and sustain such audible oscillations. This is one reason metals and alloys are useful in bells, cymbals and similar instruments.
Q48. For most materials, what happens to density as temperature increases, if mass remains unchanged?
[RRB Group D – December 2025]
(A) It never changes
(B) It always increases
(C) It usually decreases
(D) It must double
Answer: (C) It usually decreases
Explanation: Heating normally increases a substance’s volume through thermal expansion. Because density equals mass divided by volume, the same mass occupying a larger volume has lower density. Water between 0°C and 4°C is a well-known exception to this general pattern.
Q47. Which statement correctly identifies the SI unit of pressure?
[UPSC NDA-II – 2025]
(A) Pressure is a directed vector
(B) Pressure decreases with depth in water
(C) Action and reaction act on one body
(D) One pascal equals one newton per square metre
Answer: (D) One pascal equals one newton per square metre
Explanation: Pressure is normal force divided by area and is measured in pascals. Thus one pascal equals one newton per square metre. In a stationary liquid, hydrostatic pressure usually rises with depth rather than falling, making the other statements incorrect.
Q46. How does the surface tension of an ordinary liquid generally change as its temperature increases?
[UPSC NDA-II – 2025]
(A) It increases
(B) It decreases
(C) It always stays constant
(D) It decreases for either warming or cooling
Answer: (B) It decreases
Explanation: Raising temperature generally weakens the effective cohesive influence responsible for surface tension. As molecular motion increases, less work is needed to increase the liquid surface. Surface tension usually approaches zero near the liquid’s critical temperature.
Q45. A metal sphere has volume 2 × 10⁻⁴ m³ and is completely immersed in water. What buoyant force acts on it if water density is 1000 kg/m³ and g = 10 m/s²?
[UPSC CDS-I – 2025]
(A) 0.5 N
(B) 1.5 N
(C) 2 N
(D) 2.5 N
Answer: (C) 2 N
Explanation: The upward force is the weight of displaced water: F = ρVg. Substituting 1000 × 2 × 10⁻⁴ × 10 gives 2 newtons. The sphere’s own mass does not enter this buoyancy calculation when its immersed volume is known.
Q44. Which property of a liquid is related to its internal resistance to the relative motion of adjacent layers?
[SSC JE Mechanical – November 2024]
(A) Adaptability
(B) Transferability
(C) Permeability
(D) Viscosity
Answer: (D) Viscosity
Explanation: Viscosity expresses internal friction in a fluid and determines how strongly one moving layer resists sliding past another. Molecular interactions contribute to it. Honey and glycerine generally flow less readily than water because their viscosities are considerably higher.
Q43. Which pair of metal properties is particularly useful when producing thin, shiny sheets of silver for sweets?
[SSC MTS – November 2024]
(A) Ductility and lustre
(B) Malleability and sonority
(C) Lustre and sonority
(D) Malleability and lustre
Answer: (D) Malleability and lustre
Explanation: Silver can be beaten into very thin sheets because it is malleable. Its shiny appearance results from metallic lustre, which reflects visible light. Ductility instead refers to drawing wires, while sonority refers to producing a ringing sound.
Q42. What is the ability of rock or soil to allow fluids to pass through connected pores called?
[SSC MTS – October 2024]
(A) Permeability
(B) Plasticity
(C) Stability
(D) Density
Answer: (A) Permeability
Explanation: Permeability describes how readily a porous material transmits fluids through connected openings. Large or well-connected pores often increase fluid passage. Porosity measures the fraction of empty spaces and does not, by itself, establish how easily fluid can flow.
Q41. A pumpkin weighs 7.5 N and displaces 0.75 litre of water when fully immersed. Calculate its density, taking g = 10 m/s².
[UPSC NDA-II – 2024]
(A) 10 kg/m³
(B) 100 kg/m³
(C) 1000 kg/m³
(D) 10000 kg/m³
Answer: (C) 1000 kg/m³
Explanation: The pumpkin’s mass equals 7.5/10 = 0.75 kg. Its volume is 0.75 litre, or 0.00075 m³. Dividing mass by volume yields 1000 kg/m³. The displaced water volume equals its volume because the pumpkin is fully immersed.
Q40. What is the SI unit of surface tension?
[SSC JE Mechanical – June 2024]
(A) N/m²
(B) No unit
(C) N/m
(D) N/m³
Answer: (C) N/m
Explanation: Surface tension is tangential force acting per unit length along a liquid surface. Dividing the SI unit of force, newton, by the SI unit of length, metre, gives newtons per metre. It is also dimensionally equivalent to energy per area.
Q39. Which property principally makes small falling water droplets approximately spherical?
[SSC JE Civil – June 2024]
(A) Air resistance
(B) Atmospheric pressure
(C) Acceleration due to gravity
(D) Surface tension
Answer: (D) Surface tension
Explanation: Surface tension tends to minimise the surface area for a fixed liquid volume. A sphere provides the smallest surface area and therefore low surface energy. Large falling droplets may be distorted by airflow, but small droplets remain approximately spherical.
Q38. A single water drop splits into two smaller drops. Which sum is conserved?
[WBCS (Executive) Preliminary – 2023]
(A) Sum of their radii
(B) Sum of their surface areas
(C) Sum of their surface energies
(D) Sum of their volumes
Answer: (D) Sum of their volumes
Explanation: During division, water mass is conserved and its density remains essentially unchanged, so the combined volume equals the original volume. However, smaller drops expose more surface area collectively, thereby increasing total surface energy rather than conserving it.
Q37. A 40 × 20 × 10 cm block rests successively on faces measuring 20 × 10, 10 × 40 and 40 × 20 cm. How do the pressures Pₐ, Pᵦ and P꜀ compare?
[UPSC NDA-II – 2023]
(A) Pₐ > Pᵦ > P꜀
(B) Pₐ < Pᵦ < P꜀
(C) Pₐ = Pᵦ = P꜀
(D) Pₐ < Pᵦ = P꜀
Answer: (A) Pₐ > Pᵦ > P꜀
Explanation: The same block produces the same downward force in each orientation. Pressure is force divided by contact area. The three areas are respectively 200, 400 and 800 cm², so pressure falls progressively as the base area increases.
Q36. Water ascends through the xylem vessels of tall trees. Which mechanism and water properties assist this movement?
[UPSC CDS-II – 2023]
(A) Respiration and cohesion
(B) Transpiration pull, cohesion and adhesion
(C) Root pressure, cohesion and adhesion alone
(D) Transpiration and adhesion alone
Answer: (B) Transpiration pull, cohesion and adhesion
Explanation: Evaporation from leaves produces a transpiration pull that draws water upward. Cohesion keeps water molecules joined into a continuous column, while adhesion helps water interact with vessel walls. These effects jointly support upward transport through xylem in tall plants.
Q35. Which scientist is associated with the 1851 law describing the motion of a small sphere in a viscous fluid?
[SSC CHSL Tier-I – 2023]
(A) Daniel Bernoulli
(B) George Gabriel Stokes
(C) Evangelista Torricelli
(D) Heinrich Gustav Magnus
Answer: (B) George Gabriel Stokes
Explanation: George Gabriel Stokes formulated the law for viscous drag on a small sphere under suitable slow-flow conditions. Stokes’ law relates drag to the fluid’s viscosity, sphere radius and relative speed, and is useful in sedimentation and terminal-velocity problems.
Q34. Which statement about surface tension is incorrect?
[SSC CGL Tier-I – 2023]
(A) It can be expressed in dynes per centimetre
(B) It helps small water drops become spherical
(C) Water has lower surface tension than ethyl alcohol at room temperature
(D) Cohesive molecular forces cause surface tension
Answer: (C) Water has lower surface tension than ethyl alcohol at room temperature
Explanation: At ordinary room temperature, water has substantially greater surface tension than ethyl alcohol. Hydrogen bonding contributes to water’s stronger cohesive forces. The other statements correctly describe common units, the tendency toward spherical droplets and the molecular basis of surface tension.
Q33. A hollow metal sphere has density 6 g/cm³ and inner and outer radii 8 cm and 9 cm. What is its mass, taking π = 22/7?
[RRB Group D – September 2022]
(A) 4.546 kg
(B) 6.642 kg
(C) 5.456 kg
(D) 6.462 kg
Answer: (C) 5.456 kg
Explanation: The shell volume is (4/3)π(9³ − 8³) = about 909.33 cm³. Multiplying by density, 6 g/cm³, gives 5456 g, or 5.456 kg. Only the material between the inner and outer spherical surfaces contributes to its mass.
Q32. How does the density of water change when it is warmed from 0°C to 4°C?
[UPSC CDS-II – 2022]
(A) It remains constant
(B) It increases
(C) It decreases
(D) It first rises and then returns to its original value
Answer: (B) It increases
Explanation: Between 0°C and approximately 4°C, liquid water contracts upon warming and thus becomes denser. Its density reaches a maximum around 4°C. This anomalous thermal behaviour arises from changes in water’s hydrogen-bonded molecular structure.
Q31. A sealed packet has volume 1 litre and mass 800 g. Will it float in water (1 g/cm³) and in a liquid of density 1.5 g/cm³?
[UPSC NDA-II – 2022]
(A) It floats in both liquids
(B) It sinks in both liquids
(C) It sinks in water but floats in the denser liquid
(D) It floats in water but sinks in the denser liquid
Answer: (A) It floats in both liquids
Explanation: The packet’s average density is 800 g divided by 1000 cm³, or 0.8 g/cm³. Because this value is below both liquid densities, sufficient buoyant force can support it in either liquid. It therefore floats in both cases.
Q30. Hot iron can be hammered into the shape of a knife chiefly because of which property?
[SSC MTS – October 2021]
(A) Lustre
(B) Sonority
(C) Malleability
(D) Ductility
Answer: (C) Malleability
Explanation: Malleability allows a material to deform under compressive forces, such as hammering, without immediately breaking. Heated iron is easier to forge because its resistance to plastic deformation changes with temperature. Ductility instead describes forming wires by tensile drawing.
Q29. Which statement about the physical properties of metals is incorrect?
[SSC MTS – October 2021]
(A) Most metals have high melting points
(B) Metals are often good heat conductors
(C) Lead and mercury are the best heat conductors
(D) Pure metals commonly have shiny surfaces
Answer: (C) Lead and mercury are the best heat conductors
Explanation: Silver and copper are far better thermal conductors than lead and mercury. Free electrons help transfer thermal energy through metals. Although many metals conduct heat well, their conductivities differ greatly; an all-metal generalisation cannot identify lead and mercury as the best.
Q28. What phenomenon explains the absorption of ink by blotting paper?
[WBCS Preliminary – 2020]
(A) Viscosity of ink
(B) Capillary action
(C) Diffusion of ink
(D) Siphon action
Answer: (B) Capillary action
Explanation: Blotting paper contains narrow interconnected spaces through which ink spreads by capillary action. Adhesion between ink and fibres, together with liquid surface tension, drives the movement. An external pump is unnecessary, and ordinary diffusion alone does not explain the rapid uptake.
Q27. What is the SI unit of the force constant k in Hooke’s law for a spring?
[UPSC CDS-I – 2020]
(A) N·m
(B) N/m
(C) N·m²
(D) N/m²
Answer: (B) N/m
Explanation: Hooke’s law gives force magnitude F = kx, where x is extension or compression. Hence k = F/x, giving newtons per metre. The newton per square metre is a unit of pressure, while newton metre is a unit of torque or energy.
Q26. Two substances have relative densities ρ₁ and ρ₂. Their equal-volume mixture has relative density 4 and their equal-mass mixture has relative density 3. Find the two densities.
[UPSC NDA-II – 2019]
(A) 6 and 2
(B) 3 and 5
(C) 12 and 4
(D) 9 and 3
Answer: (A) 6 and 2
Explanation: Equal-volume mixing gives (ρ₁ + ρ₂)/2 = 4, so their sum is 8. Equal-mass mixing gives 2ρ₁ρ₂/(ρ₁ + ρ₂) = 3, so their product is 12. The corresponding pair is 6 and 2.
Q25. Which instrument is used to measure the pressure of a fluid?
[CGPSC Assistant Engineer – 2018]
(A) Barometer
(B) Manometer
(C) Thermometer
(D) Lactometer
Answer: (B) Manometer
Explanation: A manometer measures fluid pressure, commonly by comparing the heights of liquid columns. A barometer specifically measures atmospheric pressure. A thermometer measures temperature, whereas a lactometer estimates milk density, so those instruments do not answer the general fluid-pressure question.
Q24. For a fluid at rest in a cup, evaluate:
(1) pressure is equal at all points,
(2) pressure acts on walls,
(3) pressure exists throughout the fluid.
Which statements hold?
[UPSC NDA-I – 2018]
(A) 1 and 2 only
(B) 2 and 3 only
(C) 1 only
(D) 1, 2 and 3
Answer: (B) 2 and 3 only
Explanation: Fluid pressure exists at all points and acts normally on container walls. It need not be the same at different depths because hydrostatic pressure increases with depth. Therefore, statements about wall pressure and pressure throughout the fluid are correct.
Q23. After a wet shaving brush is withdrawn from water, why do its bristles tend to stick together?
[WBCS Preliminary – 2017]
(A) Surface tension of water
(B) Viscosity of water
(C) Elasticity of bristles
(D) Difference between air and water temperatures
Answer: (A) Surface tension of water
Explanation: Thin films of water remain between neighbouring bristles. Surface tension tends to reduce the area of those liquid surfaces, exerting forces that draw the hairs together. This is a capillary and surface-energy effect rather than a consequence of temperature difference.
Q22. What is the ratio of shear stress to shear strain within the elastic range called?
[RPSC Assistant Engineer – 2016]
(A) Bulk modulus
(B) Young’s modulus
(C) Poisson’s ratio
(D) Modulus of rigidity
Answer: (D) Modulus of rigidity
Explanation: The shear modulus, also called the modulus of rigidity, is shear stress divided by the corresponding shear strain while behaviour is elastic. Young’s modulus concerns tensile strain; bulk modulus concerns volumetric strain; Poisson’s ratio relates lateral and longitudinal strains.
Q21. Why does water rise inside a very narrow capillary tube?
[WBCS Preliminary – 2016]
(A) Elasticity
(B) Surface tension
(C) Viscosity
(D) Density alone
Answer: (B) Surface tension
Explanation: Capillary rise results from surface tension acting along a curved meniscus, together with adhesion between water and the tube. A narrower tube generally produces a higher rise. The rising column is ultimately balanced by the weight of water inside the tube.
Q20. At approximately which temperature in kelvin does pure water have its maximum density?
[UPPCS Mains – 2015]
(A) 373 K
(B) 277 K
(C) 273 K
(D) 269 K
Answer: (B) 277 K
Explanation: Water reaches maximum density near 4°C at normal atmospheric pressure. Converting Celsius to kelvin gives approximately 4 + 273 = 277 K. The phenomenon reflects anomalous expansion: water becomes less dense when cooled further toward its freezing point.
Q19. How does the pressure inside a soap bubble compare with atmospheric pressure outside it?
[UPPCS Preliminary – 2014]
(A) It is greater
(B) It is smaller
(C) It is equal
(D) It is half as large
Answer: (A) It is greater
Explanation: The curved soap film produces an excess internal pressure because surface tension pulls the film inward. For a spherical soap bubble with two surfaces, the ideal pressure difference is 4T/r. Consequently, the air pressure inside exceeds surrounding atmospheric pressure.
Q18. Without capillarity, which would be affected:
(1) using a kerosene lamp,
(2) drinking through a straw,
(3) blotting paper,
(4) growth of tall trees?
[UPSC CSE Preliminary – 2012]
(A) 1, 2 and 3 only
(B) 1, 3 and 4 only
(C) 2 and 4 only
(D) 1, 2, 3 and 4
Answer: (B) 1, 3 and 4 only
Explanation: A wick draws kerosene by capillarity, and blotting paper absorbs fluid through narrow pores. Capillarity also aids water movement in plants, alongside transpiration. Drinking with a straw chiefly depends on a pressure difference created by suction, not capillarity.
Q17. Why can a carefully placed iron needle remain supported on the surface of water?
[UPPCS Mains – 2012]
(A) Water upthrust alone
(B) Surface tension
(C) Viscosity
(D) Gravitational attraction
Answer: (B) Surface tension
Explanation: A light needle placed gently can deform the water surface without piercing it. The surface-tension force along the contact line supplies an upward component that balances its weight. Once the surface film is disturbed sufficiently, the needle may sink.
Q16. Which of these metals has the highest electrical conductivity?
[Chhattisgarh PSC Preliminary – 2011]
(A) Silver
(B) Copper
(C) Aluminium
(D) Iron
Answer: (A) Silver
Explanation: Among these pure metals at ordinary conditions, silver has the greatest electrical conductivity. Copper is widely used for wiring because it is much cheaper while remaining an excellent conductor. Conductivity depends on electron mobility and scattering within the metallic structure.
Q15. Why does kerosene normally form a floating layer on water?
[Uttarakhand Lower Subordinate Preliminary – 2010]
(A) Its density is greater than water
(B) Its density is lower than water
(C) Its density equals water
(D) None of these
Answer: (B) Its density is lower than water
Explanation: Kerosene has a lower density than water and the two liquids are largely immiscible, so kerosene generally remains above water. This arrangement minimises gravitational potential energy for the two layers and does not mean that kerosene has no mass.
Q14. Which among alcohol, water, honey and gasoline is usually the most viscous?
[Uttarakhand Lower Subordinate Preliminary – 2010]
(A) Alcohol
(B) Water
(C) Honey
(D) Gasoline
Answer: (C) Honey
Explanation: Honey generally resists flowing more strongly than alcohol, water or gasoline at similar room temperatures. Its concentrated sugars produce substantial internal resistance. Viscosity describes flow resistance and changes with temperature, so comparing substances assumes approximately the same conditions.
Q13. Why does a bucket filled with water feel heavier after it rises above the water surface in a well?
[UPPCS Preliminary – 2009]
(A) Its apparent weight increases outside water
(B) Its apparent weight decreases outside water
(C) Its stability disappears
(D) Its mass increases
Answer: (A) Its apparent weight increases outside water
Explanation: A submerged bucket receives upward buoyant force from the surrounding well water, making its apparent weight smaller. As it emerges, that support diminishes, so more lifting force is required. Its actual mass has not increased merely because of emerging.
Q12. An ice cube floats in a glass filled with water. What happens to the water level after the ice has completely melted?
[MPPSC Preliminary – 2008]
(A) It rises
(B) It falls
(C) It stays the same
(D) It rises and then falls
Answer: (C) It stays the same
Explanation: Floating ice displaces a mass of water equal to its own mass. After melting, the resulting water occupies exactly the same volume as that previously displaced, assuming pure water and ordinary conditions. Therefore, the level stays essentially unchanged.
Q11. Two soap bubbles of unequal diameter are connected by a narrow tube. What is the likely outcome?
[RAS/RTS Preliminary – 2007]
(A) Both remain unchanged
(B) The smaller shrinks and the larger grows
(C) Both become equal-sized
(D) Both burst immediately
Answer: (B) The smaller shrinks and the larger grows
Explanation: Excess pressure inside a soap bubble is inversely proportional to its radius. The smaller bubble therefore has higher internal pressure, driving air through the connecting tube into the larger bubble. It shrinks while the larger one expands.
Q10. Four wires of the same material bear equal loads. Which wire stretches the most?
[UPSC CSE Preliminary – 2007]
(A) Length 1 m, diameter 2 mm
(B) Length 2 m, diameter 2 mm
(C) Length 3 m, diameter 1.5 mm
(D) Length 1 m, diameter 1 mm
Answer: (C) Length 3 m, diameter 1.5 mm
Explanation: Elastic extension under the same load is proportional to wire length divided by its cross-sectional area, or L/d². The four relative values are 0.25, 0.50, about 1.33 and 1.00. Therefore, the third wire elongates the most.
Q9. Why can aquatic animals survive beneath a lake even when its upper surface has frozen?
[UPPCS Preliminary – 2005]
(A) They breathe inside solid ice
(B) They store enough oxygen indefinitely
(C) They require no dissolved oxygen
(D) Denser water near 4°C remains liquid below the ice
Answer: (D) Denser water near 4°C remains liquid below the ice
Explanation: Water has its maximum density near 4°C, causing that relatively warm water to settle below colder surface layers. Ice forms above and provides insulation. The lower liquid region can therefore remain habitable for aquatic organisms during severe winters.
Q8. Approximately what fraction of an iceberg remains visible above seawater?
[UPPCS Preliminary – 2005]
(A) About 1/9
(B) About 1/10
(C) About 1/6
(D) About 1/4
Answer: (A) About 1/9
Explanation: Icebergs float because ice is less dense than seawater. Approximately nine-tenths of an iceberg is submerged, while about one-tenth is exposed; the exact fraction varies with salinity and ice density. Among the supplied choices, roughly one-ninth is the keyed approximation.
Q7. A balloon of negligible weight contains 200 cm³ of water. What is its apparent weight when wholly immersed in water?
[UPSC CSE Preliminary – 2004]
(A) 9.8/5 N
(B) 9.8/10 N
(C) 9.8/2 N
(D) Zero
Answer: (D) Zero
Explanation: The balloon contains water and displaces an equal volume of surrounding water. Its downward weight is therefore exactly balanced by the buoyant force if the balloon material is weightless. Hence its apparent weight is zero, in accordance with Archimedes’ principle.
Q6. A ship passes from river water into seawater. How does its position change?
[Uttar Pradesh Lower Subordinate Preliminary – 2004]
(A) It rises slightly
(B) It remains at precisely the same level
(C) It sinks slightly
(D) It sinks to the bottom
Answer: (A) It rises slightly
Explanation: Seawater is generally denser because it contains dissolved salts. A ship requires less displaced volume to balance its unchanged weight in denser water. Consequently, it floats somewhat higher in seawater than it does in river water.
Q5. The nearly spherical shape of a small raindrop is mainly caused by which factor?
[Chhattisgarh PSC Preliminary – 2003]
(A) Liquid density
(B) Surface tension
(C) Atmospheric pressure
(D) Gravitational force
Answer: (B) Surface tension
Explanation: Surface tension encourages a liquid to minimise its surface area. For a fixed volume, the sphere has the least surface area and therefore the lowest surface energy. This explains the rounded shape of small raindrops, though airflow can distort larger ones.
Q4. A hydrogen-filled polyethylene balloon is released at ground level. What happens to its size as it rises?
[UPSC CSE Preliminary – 2003]
(A) It becomes smaller
(B) It flattens into a disc
(C) It expands
(D) Its volume stays unchanged
Answer: (C) It expands
Explanation: Atmospheric pressure decreases with altitude. The hydrogen inside the flexible balloon can therefore expand as surrounding pressure falls, assuming the envelope remains intact. Eventually expansion may be limited by the strength and elasticity of the balloon material.
Q3. Which principle is used in automobile hydraulic braking systems?
[Jharkhand PSC Preliminary – 2003]
(A) Archimedes’ principle
(B) Newton’s laws alone
(C) Bernoulli’s principle
(D) Pascal’s law
Answer: (D) Pascal’s law
Explanation: Pascal’s law states that pressure applied to a confined fluid is transmitted throughout it. Hydraulic brakes use this to transmit pressure from the pedal system to brake pistons. Different piston areas can provide useful force multiplication.
Q2. Assertion: Heating glycerine increases its viscosity.
Reason: Heating increases the kinetic energy of its molecules.
Which is correct?
[UPSC CSE Preliminary – 2002]
(A) Both true and Reason explains Assertion
(B) Both true but Reason does not explain Assertion
(C) Assertion true, Reason false
(D) Assertion false, Reason true
Answer: (D) Assertion false, Reason true
Explanation: The assertion is false because the viscosity of glycerine, like that of most liquids, decreases with increasing temperature. The reason is true: molecular kinetic energy generally rises on heating. The added motion enables molecules to rearrange and flow more readily.
Q1. Assertion: An iron ball floats on mercury but sinks in water.
Reason: Iron has a greater specific gravity than mercury.
Which option is correct?
[UPSC CSE Preliminary – 2002]
(A) Both true and Reason explains Assertion
(B) Both true but Reason does not explain Assertion
(C) Assertion true, Reason false
(D) Assertion false, Reason true
Answer: (C) Assertion true, Reason false
Explanation: The assertion is correct: iron is denser than water but less dense than mercury, so it sinks in the former and floats in the latter. The reason is false because mercury’s specific gravity is much greater than iron’s.