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.
