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#fluid mechanics

113 public questions tagged with this topic.

PHYSICS

A bubble of radius 5.5 mm is blown at 45 cm depth in water ( rho = 1000 kg/m³, S = 0.0727 N/m ). What is the total press

Given: A bubble of radius 5.5 mm is blown at 45 cm depth in water ( rho = 1000 kg/m³, S = 0.0727 N/m ). What is the total pressure inside? (Take P_a = 1.01 × 10⁵Pa, g = 10 m/s² ) Formula: P_i = P_a + rho g h + 2 S/r. Substitution & Calculation: P_a = 1.01 × 10⁵Pa, rho g h = 1000 × 10 × 0.45 = 4500 Pa . 2 S/r = frac2 × 0.07275.5 × 10⁻³= 26.44 Pa . P_i = 1.01 × 10⁵+ 4500 + 26.44 = 1.05526 × 10⁵Pa . Final Result: The computed value matches expected outcome and confirms correct choice as per latest NCERT 2026-27.

Ref: NCERT Physics Textbook - Latest Edition for Academic Session 2026-27 (Rationalized Textbook for Class XI and XII, continuing as per NCERT advisory for 2026-27),Topic: Fundamental laws, definitions and applications as per latest NCERT. The section explains governing laws, formulas like μ₀ = 4π.

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PHYSICS

A manometer with water ( rho = 1000 kg/m³ ) shows a height difference of 0.15 m . What is the pressure difference? (Take

Given: A manometer with water ( rho = 1000 kg/m³ ) shows a height difference of 0.15 m . What is the pressure difference? (Take g = 9.8 m/s² ) Formula: Δ P = rho g h. Substitution & Calculation: rho = 1000 kg/m³, g = 9.8 m/s², h = 0.15 m . Δ P = 1000 × 9.8 × 0.15 = 1470 Pa . Final Result: The computed value matches expected outcome and confirms correct choice as per latest NCERT 2026-27.

Ref: NCERT Physics Textbook - Latest Edition for Academic Session 2026-27 (Rationalized Textbook for Class XI and XII, continuing as per NCERT advisory for 2026-27),Topic: Fundamental laws, definitions and applications as per latest NCERT. The section explains governing laws, formulas like μ₀ = 4π.

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PHYSICS

What is the reason a liquid’s terminal velocity depends on its radius when falling through a viscous medium?

Stokes’ law ( F = 6 π eta a v ) shows that viscous force is proportional to radius ( a ), while weight ( 4/3 π a³ rho g ) depends on a³ . At terminal velocity, these balance, making v propto a² .

Ref: NCERT Physics Textbook for Class XI and XII, Chapter: Laws of Motion, Work Energy Power, Gravitation and System of Particles, Topic: Newton's laws, work-energy theorem and rotational dynamics.

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Fluid Pressure, Pascal's Law and Manometers

What is the physical significance of the term 12ρv2 in Bernoulli’s equation?

In Bernoulli’s equation, 12ρv2 represents the kinetic energy per unit volume of the fluid, reflecting the energy associated with its motion, balanced against pressure and potential energy terms. As per NCERT, applying relevant law/formula with correct units and sign convention leads to Kinetic energy per unit volume. This satisfies dimensional consistency and physical conditions given, so option C is scientifically correct.

Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.

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Fluid Pressure, Pascal's Law and Manometers

A manometer with mercury (ρ\=13.6×103kg/m3) shows a height difference of 20cm. What is the pressure difference? (Take g\

ΔP = ρgh. ρ = 13.6×103kg/m3, g = 9.8m/s2, h = 0.2m. ΔP = 13.6×103×9.8×0.2 = 26656Pa. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 2.67 × 10⁴ Pa. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.

Ref: NCBI Bookshelf, Ideal Gas Law: P1V1/T1 = P2V2/T2.

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Surface Tension, Capillarity and Excess Pressure

Which of the following statements is correct about atmospheric pressure?

Atmospheric pressure decreases with altitude as the weight of the air column above reduces, a fundamental concept in the chapter’s discussion of pressure variation. As per NCERT, applying relevant law/formula with correct units and sign convention leads to It decreases with altitude. This satisfies dimensional consistency and physical conditions given, so option C is scientifically correct.

Ref: NCBI Bookshelf, Ideal Gas Law: P1V1/T1 = P2V2/T2.

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Surface Tension, Capillarity and Excess Pressure

Why does the viscosity of liquids generally decrease with increasing temperature?

Higher temperatures increase molecular kinetic energy in liquids, weakening intermolecular forces, reducing resistance to flow, and thus lowering viscosity, as noted in the chapter. As per NCERT, applying relevant law/formula with correct units and sign convention leads to Intermolecular forces weaken. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.

Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.

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Surface Tension, Capillarity and Excess Pressure

What is the force on a submarine window (0.06m2) at 400m depth in seawater (ρ\=1.03×103kg/m3), interior at atmospheric p

Gauge pressure: Pg = ρgh = 1.03×103×9.8×400 = 4.0376×106Pa. F = PgA = 4.0376×106×0.06 = 2.42256×105N. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 2.4 × 10⁵ N. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.

Ref: NCBI Bookshelf, Ideal Gas Law: P1V1/T1 = P2V2/T2.

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Surface Tension, Capillarity and Excess Pressure

What is the excess pressure inside a mercury drop of radius 4mm at 20∘C? (Surface tension = 0.4355N/m)

Excess pressure: ΔP = 2Sr. S = 0.4355N/m, r = 4×10−3m. ΔP = 2×0.43554×10−3 = 217.75Pa. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 218 Pa. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.

Ref: NCBI Bookshelf, Ideal Gas Law: P1V1/T1 = P2V2/T2.

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Surface Tension, Capillarity and Excess Pressure

Why does a liquid’s surface tension decrease with increasing temperature?

Increasing temperature weakens cohesive forces between liquid molecules by increasing their kinetic energy, reducing the contractile force at the surface, thus lowering surface tension, as noted in the chapter. As per NCERT, applying relevant law/formula with correct units and sign convention leads to Cohesive forces weaken. This satisfies dimensional consistency and physical conditions given, so option C is scientifically correct.

Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.

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Temperature Scales and Triple Point

Which statement is incorrect regarding Torricelli’s law?

Torricelli’s law (v = 2gh) assumes non-viscous flow and neglects losses, not depending on viscosity or requiring turbulence. The incorrect statement is that it depends on fluid viscosity. As per NCERT, applying relevant law/formula with correct units and sign convention leads to It depends on fluid viscosity. This satisfies dimensional consistency and physical conditions given, so option C is scientifically correct.

Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.

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