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.
