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.
