Mechanical Properties of Fluids – Fluid's Flow: NEET MCQs with Answers

15 free questions on Mechanical Properties of Fluids – Fluid's Flow for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Fluid's Flow, Viscosity and Bernoulli's Principle: Viscosity. Stokes' law. terminal velocity, streamline, and turbulent flow. critical velocity. Bernoulli's principle and its applications.

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Question 1

Previous-year question
A body is falling freely in a resistive medium. Its motion is described by dv/dt = 4 − 2v, where v (in m s⁻¹) is its velocity at any instant. The initial acceleration and the terminal velocity are respectively: [NEET 2024]
  1. 2 m s⁻², 4 m s⁻¹
  2. 6 m s⁻², 2 m s⁻¹
  3. 2 m s⁻², 6 m s⁻¹
  4. 4 m s⁻², 2 m s⁻¹
Show answer
Answer: (D) 4 m s⁻², 2 m s⁻¹
Explanation: Initially v = 0, so a = dv/dt = 4 m s⁻². At terminal velocity dv/dt = 0 ⇒ 4 − 2v = 0 ⇒ v = 2 m s⁻¹.

Question 2

Previous-year question
The viscous drag acting on a metal sphere of diameter 1 mm, falling through a fluid of viscosity 0.8 Pa·s with a velocity of 2 m s⁻¹, is: [NEET 2023]
  1. 15 × 10⁻³ N
  2. 30 × 10⁻³ N
  3. 1.5 × 10⁻³ N
  4. 20 × 10⁻³ N
Show answer
Answer: (A) 15 × 10⁻³ N
Explanation: r = 0.5 mm = 5 × 10⁻⁴ m. F = 6πηrv = 6 × 3.14 × 0.8 × 5 × 10⁻⁴ × 2 ≈ 15 × 10⁻³ N.

Question 3

Previous-year question
The terminal velocity of a copper ball of radius 5 mm falling through a tank of oil at room temperature is 10 cm s⁻¹. If the viscosity of the oil is 0.9 kg m⁻¹ s⁻¹, the viscous drag force is: [NEET 2022]
  1. 8.48 × 10⁻³ N
  2. 8.48 × 10⁻⁵ N
  3. 4.23 × 10⁻³ N
  4. 4.23 × 10⁻⁶ N
Show answer
Answer: (A) 8.48 × 10⁻³ N
Explanation: F = 6πηrv = 6 × 3.14 × 0.9 × 5 × 10⁻³ × 0.1. F = 8.48 × 10⁻³ N.

Question 4

Previous-year question
The correct statement about the variation of viscosity of a fluid with increase in temperature is: [NEET 2022]
  1. viscosity of both liquids and gases increases
  2. viscosity of liquids increases
  3. viscosity of liquids decreases
  4. viscosity of gases decreases
Show answer
Answer: (C) viscosity of liquids decreases
Explanation: In liquids, viscosity is due to cohesive forces, which weaken on heating, so η decreases. In gases, viscosity arises from molecular transfer between layers, which increases with temperature.

Question 5

Previous-year question
An open rectangular vessel full of petrol takes 20√2 minutes to empty through an orifice in its bottom. The time it takes to empty when half filled with petrol is: (neglect evaporation) [NEET 2021]
  1. 10 min
  2. 15 min
  3. 20 min
  4. 25 min
Show answer
Answer: (C) 20 min
Explanation: Time to empty a tank through a small orifice t = (A/a)√(2H/g) ∝ √H. t′ = 20√2 × √(1/2) = 20 min.

Question 6

Previous-year question
An ideal fluid flows in a non-uniform tube XY (as shown) from end X to end Y. If K₁ and K₂ are the kinetic energies per unit volume of the fluid at X and Y respectively, the correct option is: [Re-NEET 2024]
Figure for question 6
  1. K₁ = K₂
  2. 2K₁ = K₂
  3. K₁ > K₂
  4. K₁ < K₂
Show answer
Answer: (C) K₁ > K₂
Explanation: By continuity Av = constant, so the fluid moves faster where the tube is narrower. Kinetic energy per unit volume ½ρv² is therefore larger at the narrower end X than at the wider end Y: K₁ > K₂.

Question 7

Previous-year question
A fluid of density ρ is flowing in a pipe of varying cross-sectional area as shown. Bernoulli's equation for the motion becomes: [NEET 2022]
Figure for question 7
  1. p + ½ρv² = constant
  2. ½ρv² + ρgh = constant
  3. p + ρgh = constant
  4. p + ½ρv² + ρgh = constant
Show answer
Answer: (D) p + ½ρv² + ρgh = constant
Explanation: The pipe changes both in cross-section (so v changes) and in height (so h changes). Hence the full form p + ½ρv² + ρgh = constant is needed.
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Question 8

Previous-year question
A small hole of area of cross-section 2 mm² is present near the bottom of a fully filled open tank of height 2 m. Taking g = 10 m/s², the rate of flow of water through the open hole would be nearly [NEET 2019]
  1. 12.6 × 10⁻⁶ m³/s
  2. 8.9 × 10⁻⁶ m³/s
  3. 2.23 × 10⁻⁶ m³/s
  4. 6.4 × 10⁻⁶ m³/s
Show answer
Answer: (A) 12.6 × 10⁻⁶ m³/s
Explanation: v = √(2gh) = √40 ≈ 6.32 m/s, so Q = Av = 2 × 10⁻⁶ × 6.32 ≈ 12.6 × 10⁻⁶ m³/s.

Question 9

Previous-year question
A small sphere of radius r falls from rest in a viscous liquid. As a result, heat is produced due to the viscous force. The rate of production of heat when the sphere attains its terminal velocity is proportional to [NEET 2018]
  1. r³
  2. r²
  3. r⁴
  4. r⁵
Show answer
Answer: (D) r⁵
Explanation: The rate of heat production is F v_t = (6πηrv_t)v_t ∝ r × (r²)² = r⁵, since v_t ∝ r².

Question 10

Previous-year question
The terminal velocity v_r of a small steel ball of radius r falling under gravity through a column of a viscous liquid depends on the mass of the ball m, the acceleration due to gravity g, the coefficient of viscosity η and the radius r. Which of the following relations is dimensionally correct? [AIPMT 1992]
  1. v_r ∝ mgr/η
  2. v_r ∝ mgηr
  3. v_r ∝ mg/(rη)
  4. v_r ∝ ηmg/r
Show answer
Answer: (C) v_r ∝ mg/(rη)
Explanation: At terminal velocity, Stokes' law gives the balance mg = 6πηr v_r, so v_r = mg/(6πηr) ⟹ v_r ∝ mg/(rη). A dimensional check confirms this: [mg/(rη)] = (M L T⁻²)/(L × M L⁻¹ T⁻¹) = L T⁻¹, which is indeed a velocity.  

Question 11

Previous-year question
A fluid is in streamline flow across a horizontal pipe of variable cross-sectional area. For this, which of the following statements is correct? [NEET 2013]
  1. The velocity is minimum at the narrowest part of the pipe and the pressure is minimum at the widest part of the pipe
  2. The velocity is maximum at the narrowest part of the pipe and the pressure is maximum at the widest part of the pipe
  3. Velocity and pressure are both maximum at the narrowest part of the pipe
  4. Velocity and pressure are both maximum at the widest part of the pipe
Show answer
Answer: (B) The velocity is maximum at the narrowest part of the pipe and the pressure is maximum at the widest part of the pipe
Explanation: By the equation of continuity, Av = constant, so the speed is greatest where the cross-section is smallest — the velocity is maximum at the narrowest part. By Bernoulli's theorem for a horizontal pipe, p + ½ρv² = constant, so the pressure is greatest where the speed is least — that is at the widest part. Hence velocity is maximum at the narrowest section while pressure is maximum at the widest section.

Question 12

Previous-year question
The cylindrical tube of a spray pump has radius R, one end of which has n fine holes each of radius r. If the speed of the liquid in the tube is v, the speed of ejection of the liquid through the holes is [AIPMT 2015]
  1. vR²/(n²r²)
  2. vR²/(nr²)
  3. vR²/(n³r²)
  4. v²R/(nr)
Show answer
Answer: (B) vR²/(nr²)
Explanation: By the equation of continuity, the volume flow rate in the tube equals the total flow rate through all the holes: Av = A′v′ πR²v = n(πr²)v′ v′ = R²v/(nr²). Since the holes are fine and numerous, the emerging jet is much faster than the flow inside the tube.

Question 13

Previous-year question
A wind with a speed of 40 m/s blows parallel to the roof of a house. The area of the roof is 250 m². Assuming that the pressure inside the house is atmospheric pressure, the force exerted by the wind on the roof and its direction will be (ρ_air = 1.2 kg/m³) [AIPMT 2015]
  1. 4.8 × 10⁵ N, downwards
  2. 4.8 × 10⁵ N, upwards
  3. 2.4 × 10⁵ N, upwards
  4. 2.4 × 10⁵ N, downwards
Show answer
Answer: (C) 2.4 × 10⁵ N, upwards
Explanation: By Bernoulli's theorem, applied to the still air inside and the moving air outside: p₁ − p₂ = ½ρ(v₂² − v₁²) = ½ × 1.2 × (40² − 0) = 960 N/m². F = (p₁ − p₂)A = 960 × 250 = 2.4 × 10⁵ N. The pressure inside is greater than outside, so the net force pushes the roof upwards — this is why roofs are blown off in a storm.

Question 14

Previous-year question
A small sphere of radius r falls from rest in a viscous liquid. As a result, heat is produced due to the viscous force. The rate of production of heat when the sphere attains its terminal velocity is proportional to [NEET 2018]
  1. r⁵
  2. r²
  3. r³
  4. r⁴
Show answer
Answer: (A) r⁵
Explanation: The rate of heat production is the power dissipated against the viscous drag: dQ/dt = F × v_T = (6πηr v_T) × v_T = 6πηr v_T². The terminal velocity itself satisfies v_T = 2r²(ρ − σ)g/(9η), so v_T ∝ r² and hence v_T² ∝ r⁴. dQ/dt ∝ r × r⁴ = r⁵.

Question 15

Previous-year question
A small hole of cross-sectional area 2 mm² is present near the bottom of a fully filled open tank of height 2 m. Taking g = 10 m/s², the rate of flow of water through the open hole would be nearly [NEET (National) 2019]
  1. 8.9 × 10⁻⁶ m³/s
  2. 2.23 × 10⁻⁶ m³/s
  3. 6.4 × 10⁻⁶ m³/s
  4. 12.6 × 10⁻⁶ m³/s
Show answer
Answer: (D) 12.6 × 10⁻⁶ m³/s
Explanation: By Torricelli's law the efflux speed is v = √(2gh) = √(2 × 10 × 2) = √40 = 6.32 m/s. Rate of flow = area × velocity = (2 × 10⁻⁶ m²) × 6.32 = 12.64 × 10⁻⁶ ≈ 12.6 × 10⁻⁶ m³/s.

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