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Mechanical Properties of Fluids — NEET & JEE MCQs with answers
This chapter studies the static and dynamic behaviour of fluids (liquids and gases), covering pressure, Pascal's law, Bernoulli's principle, viscosity (Stokes' law), and surface tension. Key phenomena include hydrostatic pressure variation with depth, hydraulic machines, streamline and turbulent flow, capillary rise, and the Magnus effect.
Topics in this chapter
- 9.1 Introduction
- 9.2 Pressure
- 9.3 Streamline Flow
- 9.4 Bernoulli's Principle
- 9.5 Viscosity
- 9.6 Surface Tension
Free practice questions
At what depth in sea water ($\rho=1.03\times10^{3}\,\text{kg m}^{-3}$, $g=10\,\text{m s}^{-2}$) does the gauge pressure reach $2.06\times10^{5}\,\text{Pa}$?
- A. $2\,\text{m}$
- B. $200\,\text{m}$
- C. $20\,\text{m}$
- D. $10\,\text{m}$
Show answer
Answer: C — $h=P/(\rho g)=2.06\times10^{5}/(1.03\times10^{3}\times10)=2.06\times10^{5}/(1.03\times10^{4})=20\,\text{m}$.
NCERT Class 11 Physics · Ch 9 · 9.2 Pressure
Water flows steadily through a horizontal pipe. At a wide section (area 40 cm²) the speed is 0.5 m/s, and at a narrow section the area is 8 cm². The speed at the narrow section is:
- A. 2.5 m/s
- B. 0.1 m/s
- C. 0.5 m/s
- D. 5 m/s
Show answer
Answer: A — By continuity A₁v₁ = A₂v₂, so v₂ = (40 × 0.5)/8 = 20/8 = 2.5 m/s.
NCERT Class 11 Physics · Ch 9 · 9.3 Streamline Flow
In level flight, the dynamic lift on an aircraft's wings balances
- A. The weight of the plane
- B. The atmospheric pressure
- C. The thrust of the engines
- D. The air drag
Show answer
Answer: A — NCERT: 'There is an upward force resulting in a dynamic lift of the wings and this balances the weight of the plane.'
NCERT Class 11 Physics · Ch 9 · 9.4 Bernoulli's Principle
A sphere falls through a liquid with terminal velocity v_t. If the coefficient of viscosity of the liquid is DOUBLED (all else unchanged), the new terminal velocity is:
- A. 4 v_t
- B. v_t / 4
- C. v_t / 2
- D. 2 v_t
Show answer
Answer: C — v_t is proportional to 1/eta. Doubling eta halves the terminal velocity. New terminal velocity = v_t / 2.
NCERT Class 11 Physics · Ch 9 · 9.5 Viscosity
A mercury drop of radius $2\ mm$ has surface tension $0.4355\ N/m$. The excess pressure inside is about:
- A. $217.75\ Pa$
- B. $871\ Pa$
- C. $435.5\ Pa$
- D. $108.9\ Pa$
Show answer
Answer: C — P=2S/r=2(0.4355)/(2e-3)=0.871/2e-3=435.5 Pa.
NCERT Class 11 Physics · Ch 9 · 9.6 Surface Tension
The volume of a solid, liquid or gas depends on:
- A. only its electrical charge
- B. nothing; it is absolutely fixed
- C. only its colour
- D. the stress or pressure acting on it
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Answer: D — NCERT states that the volume of solid, liquid or gas depends on the stress or pressure acting on it.
NCERT Class 11 Physics · Ch 9 · 9.1 Introduction
Two syringes are connected by a tube filled with water. Diameters of the smaller and larger pistons are $1.0\,\text{cm}$ and $3.0\,\text{cm}$. A force of $10\,\text{N}$ on the smaller piston produces a force on the larger piston of
- A. $30\,\text{N}$
- B. $10\,\text{N}$
- C. $90\,\text{N}$
- D. $3.3\,\text{N}$
Show answer
Answer: C — Force ratio = area ratio = $(d_2/d_1)^2=(3/1)^2=9$. $F_2=10\times9=90\,\text{N}$ (Example 9.5a).
NCERT Class 11 Physics · Ch 9 · 9.2 Pressure
When the speed of outflow from a tap is increased sufficiently, the smooth flow:
- A. loses its smoothness
- B. becomes perfectly steady
- C. stops completely
- D. remains exactly the same
Show answer
Answer: A — NCERT 9.3 states that when a water tap is turned on slowly the flow is smooth, but it loses its smoothness when the speed of the outflow is increased.
NCERT Class 11 Physics · Ch 9 · 9.3 Streamline Flow
A large open water tank has a small hole at depth 11.25 m below the surface. Neglecting viscosity, the efflux speed is (g = 10 m/s²)
- A. 7.5 m/s
- B. 11.25 m/s
- C. 15 m/s
- D. 22.5 m/s
Show answer
Answer: C — v = √(2gh) = √(2 × 10 × 11.25) = √225 = 15 m/s.
NCERT Class 11 Physics · Ch 9 · 9.4 Bernoulli's Principle
The terminal velocity v_t of a small sphere falling through a viscous fluid is proportional to which power of its radius a?
- A. a^3
- B. a^2
- C. a^(1/2)
- D. a
Show answer
Answer: B — From v_t = 2 a^2 (rho - sigma) g / (9 eta), terminal velocity depends on a^2. Hence v_t is proportional to the SQUARE of the radius (v_t proportional to a^2).
NCERT Class 11 Physics · Ch 9 · 9.5 Viscosity
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