JEE Physics Mechanical Properties Of Fluids MCQs Set 36

Multiple Choice Questions (MCQs) for JEE Physics: Mechanical Properties Of Fluids

Review structured MCQ sets for JEE Physics Mechanical Properties Of Fluids. Built according to official JEE guidelines, these downloadable questions support daily revision and core concept reinforcement.

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Multiple Choice Questions

Question 1: Match Column I with Column II.
Column IColumn II
(A) Barometer(1) Law of conservation of energy
(B) Hydrometer(2) To measure density
(C) Bernoulli's principle(3) To measure atmospheric pressure
(D) Archimedes' principle(4) Upthrust

(a) A-3, B-1, C-2, D-4
(b) A-2, B-3, C-1, D-4
(c) A-3, B-2, C-1, D-4
(d) A-3, B-4, C-1, D-2
Show Answer & Explanation

Answer: (c) A-3, B-2, C-1, D-4

Explanation:
1. A barometer measures atmospheric pressure (A-3).
2. A hydrometer measures density (B-2).
3. Bernoulli's principle is energy conservation (C-1).
4. Archimedes' principle deals with upthrust (D-4).

Teacher's Note:
1. Match instruments with their uses.

Question 2: A solid ball of volume V experiences a viscous force F when falling with a speed v in a liquid. If another ball of volume 8V is allowed to fall in the same liquid with the same velocity v, it experiences a force
(a) F
(b) 16F
(c) 4F
(d) 2F
Show Answer & Explanation

Answer: (d) 2F

Explanation:
1. F = 6πηrv.
2. The volume 8V has a radius twice as large, so F′ = 2F.

Teacher's Note:
1. Stokes' drag is proportional to r.

Question 3: Streamline flow is more likely for liquids with
(a) high density and low viscosity
(b) low density and high viscosity
(c) high density and high viscosity
(d) low density and low viscosity
Show Answer & Explanation

Answer: (b) low density and high viscosity

Explanation:
1. Reynolds number Re = ρvD/η.
2. Low density and high viscosity make Re small, favouring streamline flow.

Teacher's Note:
1. Small Re means laminar flow.

Question 4: Which of the following statement(s) is/are true? I. For gases, in general, viscosity increases with temperature. II. For liquids, viscosity varies directly with pressure. III. For gases, viscosity is independent of pressure.
(a) I and II
(b) II and III
(c) I and III
(d) I, II and III
Show Answer & Explanation

Answer: (c) I and III

Explanation:
1. Gas viscosity increases with temperature and is almost independent of pressure.
2. The viscosity of liquids generally decreases (not increases) with rising pressure for water, so II is not a general truth.

Teacher's Note:
1. Kinetic theory gives η ∝ √T for gases.

Question 5:
Assertion (A): Lifting of an aircraft is caused by a pressure difference brought about by varying the speed of air molecules.
Reason (R): As the wings/aerofoils move against the wind, the streamlines crowd above them more than below them, causing higher velocity above.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Higher speed above lowers the pressure above the wing relative to below (Bernoulli).
2. The crowded streamlines above explain the higher velocity.

Teacher's Note:
1. Bernoulli lift.

Question 6:
Assertion (A): Mercury is preferred as a barometric substance over water.
Reason (R): Mercury is opaque and shiny, so it is easier to note the observation.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Mercury's large density gives a short column of about 76 cm, which is the main reason.
2. Its opacity is a true but secondary fact, so the reason does not explain the assertion.

Teacher's Note:
1. High density keeps the barometer short.

Question 7: If two soap bubbles of different radii are in communication with each other then
(a) air flows from the larger bubble into the smaller one until the two bubbles are of equal size
(b) the size of the bubbles remains the same
(c) air flows from the smaller bubble into the larger one and the larger one grows at the expense of the smaller one
(d) air flows from the larger bubble into the smaller bubble until the radius of the smaller one becomes equal to that of the larger one
Show Answer & Explanation

Answer: (c) air flows from the smaller bubble into the larger one and the larger one grows at the expense of the smaller one

Explanation:
1. The excess pressure 4s/r is higher in the smaller bubble.
2. So air flows from the smaller bubble into the larger one and the smaller bubble shrinks.

Teacher's Note:
1. Smaller bubble has more pressure.

Question 8: As the temperature of a liquid is raised, the coefficient of viscosity
(a) decreases
(b) increases
(c) remains the same
(d) may increase or decrease depending on the nature of the liquid
Show Answer & Explanation

Answer: (a) decreases

Explanation:
1. Molecular cohesion weakens with heating.
2. The viscosity of a liquid decreases.

Teacher's Note:
1. Hot liquids flow more easily.

Question 9: Beyond the critical speed, the flow of a fluid becomes
(a) streamline
(b) turbulent
(c) steady
(d) very slow
Show Answer & Explanation

Answer: (b) turbulent

Explanation:
1. Above the critical speed (Re above about 3000), the flow breaks into eddies.
2. The flow is turbulent.

Teacher's Note:
1. Turbulence appears at high speed.

Question 10: The total weight of a piece of wood is 6 kg. In the floating state in water, 1/3 of it remains inside the water. What maximum weight is to be put on this floating piece of wood so that the whole of the piece of wood is to be drowned in the water?
(a) 15 kg
(b) 14 kg
(c) 10 kg
(d) 12 kg
Show Answer & Explanation

Answer: (d) 12 kg

Explanation:
1. Floating: 6 kg = weight of water displaced by V/3, so the weight of water of volume V is 18 kg.
2. For full submersion, 6 + W = 18, so W = 12 kg.

Teacher's Note:
1. Total buoyancy equals total weight.

Question 11: If a ball of steel (density ρ = 7.8 g cm⁻³) attains a terminal velocity of 10 cm s⁻¹ when falling in a tank of water (coefficient of viscosity η_water = 8.5 × 10⁻⁴ Pa s), then its terminal velocity in glycerine (ρ = 1.2 g cm⁻³, η = 13.2 Pa s) would be nearly
(a) \( 1.6\times10^{-5}\ \text{cm s}^{-1} \)
(b) \( 6.25\times10^{-4}\ \text{cm s}^{-1} \)
(c) \( 6.45\times10^{-4}\ \text{cm s}^{-1} \)
(d) \( 1.5\times10^{-5}\ \text{cm s}^{-1} \)
Show Answer & Explanation

Answer: (b) \( 6.25\times10^{-4}\ \text{cm s}^{-1} \)

Explanation:
1. vₜ ∝ (ρ − σ)/η.
2. v = 10 × [(7.8 − 1.2)/13.2] / [(7.8 − 1)/(8.5 × 10⁻⁴)] = 10 × 0.5/8000 = 6.25 × 10⁻⁴ cm/s.

Teacher's Note:
1. Use the ratio from Stokes' law.

Question 12: An open glass tube is immersed in mercury in such a way that a length of 8 cm extends above the mercury level. The open end of the tube is then closed and sealed and the tube is raised vertically up by an additional 46 cm. What will be the length of the air column above mercury in the tube now? (Atmospheric pressure = 76 cm of Hg)
(a) 16 cm
(b) 22 cm
(c) 38 cm
(d) 6 cm
Show Answer & Explanation

Answer: (a) 16 cm

Explanation:
1. Let h be the mercury column raised above the outer level.
2. Air length L = 8 + 46 − h and pressure = 76 − h.
3. 76 × 8 = (76 − h)(54 − h) gives h = 38 cm, so L = 16 cm.

Teacher's Note:
1. Use Boyle's law with the changed column.

Question 13: Which of the following statements are true about streamline flow? I. The path taken by a fluid particle under a steady flow is a streamline. II. No two streamlines can cross each other. III. The velocity increases at the narrower portions where the streamlines are closely spaced.
(a) I and II only
(b) II and III only
(c) I and III only
(d) I, II and III
Show Answer & Explanation

Answer: (d) I, II and III

Explanation:
1. All three are properties of streamlines.
2. The streamline spacing is inversely proportional to the speed.

Teacher's Note:
1. Streamlines never cross.

Question 14: The most characteristic property of a liquid is
(a) elasticity
(b) fluidity
(c) formlessness
(d) volume conservation
Show Answer & Explanation

Answer: (b) fluidity

Explanation:
1. A liquid flows and takes the shape of its container.
2. The most characteristic property is fluidity.

Teacher's Note:
1. Liquids flow.

Question 15:
Assertion (A): If a body is floating in a liquid, the density of the liquid is always greater than or equal to the density of the solid.
Reason (R): Surface tension is the property of a liquid surface.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A body floats only if its average density does not exceed that of the liquid, so the assertion is true.
2. The reason is a true statement but unrelated to the assertion.

Teacher's Note:
1. Both true, unrelated.

Question 16: If more air is pushed into a soap bubble, the pressure in it
(a) decreases
(b) becomes zero
(c) remains the same
(d) increases
Show Answer & Explanation

Answer: (a) decreases

Explanation:
1. The bubble radius increases when more air is pushed in.
2. Excess pressure 4s/r falls as r grows, so the pressure decreases.

Teacher's Note:
1. Larger bubble has lower excess pressure.

Question 17: According to Stokes' law, the relation between the terminal velocity (vₜ) and the viscosity of the medium (η) is
(a) \( v_t=\eta \)
(b) \( v_t\propto\eta \)
(c) \( v_t\propto\frac1\eta \)
(d) \( v_t\ \text{is independent of}\ \eta \)
Show Answer & Explanation

Answer: (c) \( v_t\propto\frac1\eta \)

Explanation:
1. vₜ = 2r²(ρ − σ)g/9η.
2. So vₜ is inversely proportional to η.

Teacher's Note:
1. More viscous means slower fall.

Question 18:
Assertion (A): The concept of surface tension is held only for liquids.
Reason (R): Surface tension does not hold for gases.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A free boundary surface with unbalanced cohesive forces exists only for liquids.
2. Gases fill their container and have no free surface, so the reason explains the assertion.

Teacher's Note:
1. Gases have no free surface.

Question 19: The device which measures the flow speed of an incompressible fluid is
(a) sphygmomanometer
(b) open-tube manometer
(c) venturimeter
(d) mercury barometer
Show Answer & Explanation

Answer: (c) venturimeter

Explanation:
1. A venturimeter uses the pressure difference between a wide and a narrow section.
2. It gives the flow speed.

Teacher's Note:
1. Venturi meter measures flow speed.

Question 20:
Assertion (A): Imagine holding two identical bricks under water. Brick A is completely submerged just below the surface of water, while brick B is at a greater depth. The magnitude of force exerted by the person to hold brick B in place is the same as the magnitude of the force exerted to hold brick A in place.
Reason (R): The magnitude of the buoyant force on a brick completely submerged in water is equal to the magnitude of the weight of water it displaces and does not depend on the depth of the brick in water.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The buoyant force depends only on the displaced volume, not on depth.
2. So the holding force is the same, and the reason explains it.

Teacher's Note:
1. Buoyancy is depth independent.

Question 21: With the increase in temperature, the angle of contact
(a) decreases
(b) increases
(c) remains constant
(d) sometimes increases and sometimes decreases
Show Answer & Explanation

Answer: (a) decreases

Explanation:
1. Surface tension decreases with temperature, so cos θ = (S_sa − S_sl)/S_la increases.
2. So θ decreases.

Teacher's Note:
1. θ decreases on heating.

Question 22: Water rises to a height h in a capillary tube. If the length of the capillary tube above the surface of water is made less than h, then
(a) water rises up to the top of the capillary tube and stays there without overflowing
(b) water rises up to a point a little below the top and stays there
(c) water does not rise at all
(d) water rises up to the tip of the tube and then starts overflowing like a fountain
Show Answer & Explanation

Answer: (a) water rises up to the top of the capillary tube and stays there without overflowing

Explanation:
1. The meniscus radius adjusts so that R × h stays constant.
2. The liquid rises to the top, the meniscus flattens and no overflow occurs.

Teacher's Note:
1. The meniscus changes its curvature.

Question 23: Which pressure-measuring device measures the gauge pressure?
(a) Mercury barometer
(b) Sphygmomanometer
(c) Both (a) and (b)
(d) None of these
Show Answer & Explanation

Answer: (b) Sphygmomanometer

Explanation:
1. A barometer measures the atmospheric pressure.
2. A sphygmomanometer measures blood pressure relative to atmospheric pressure, which is a gauge pressure.

Teacher's Note:
1. Gauge pressure is relative to atmosphere.

Question 24: When the temperature increases, the viscosity of
(a) gases decreases and liquids increases
(b) gases increases and liquids decreases
(c) gases and liquids increases
(d) gases and liquids decreases
Show Answer & Explanation

Answer: (b) gases increases and liquids decreases

Explanation:
1. In gases viscosity comes from momentum transfer by faster molecules, which increases with temperature.
2. In liquids it comes from cohesive forces, which weaken on heating.

Teacher's Note:
1. Opposite trends for gases and liquids.

Question 25: The radius of a capillary tube is 2 × 10⁻³ m. A liquid of weight 6.28 × 10⁻⁴ N may remain in the capillary; then the surface tension of the liquid will be
(a) \( 5\times10^3\ \text{N/m} \)
(b) \( 5\times10^{-2}\ \text{N/m} \)
(c) \( 5\ \text{N/m} \)
(d) \( 50\ \text{N/m} \)
Show Answer & Explanation

Answer: (b) \( 5\times10^{-2}\ \text{N/m} \)

Explanation:
1. T × 2πr = W.
2. T = 6.28 × 10⁻⁴/(2π × 2 × 10⁻³) = 5 × 10⁻² N/m.

Teacher's Note:
1. Balance the weight with the surface-tension force.

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