Mechanical Properties of Fluids — 50 High-Yield MCQs (NCERT / NEET)

Covers density & pressure, hydrostatics, buoyancy, viscosity, surface tension, Bernoulli's principle, Poiseuille flow, Reynolds number, and key formulas. Correct answer shown after each question.

50 MCQs
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Hydrostatics Buoyancy Viscosity Dynamics & Bernoulli
1. Density (ρ) of a substance is defined as:
A. Mass × volume
B. Mass / volume
C. Volume / mass
D. Mass + volume
Answer: B
2. Hydrostatic pressure at depth h in a liquid of density ρ is:
A. ρ g h
B. ρ g / h
C. ρ / (g h)
D. g h / ρ
Answer: A
3. Pascal’s law states that pressure applied to a confined fluid is:
A. Transmitted undiminished in all directions
B. Decreases with distance
C. Only downward
D. Zero
Answer: A
4. Atmospheric pressure at sea level is approximately:
A. 1.01 × 10^5 Pa
B. 1.01 × 10^3 Pa
C. 9.8 × 10^1 Pa
D. 1 Pa
Answer: A
5. Gauge pressure is defined as:
A. Absolute pressure − atmospheric pressure
B. Absolute pressure + atmospheric pressure
C. Atmospheric pressure only
D. Zero
Answer: A
6. Buoyant force on a body immersed in fluid equals:
A. Weight of displaced fluid
B. Weight of body
C. Volume of body
D. Density × volume of body
Answer: A
7. An object floats when its average density is:
A. Greater than fluid density
B. Equal to fluid density
C. Less than fluid density
D. Infinity
Answer: C
8. Archimedes' principle is used to determine:
A. Density of fluids and buoyant force
B. Viscosity only
C. Surface tension only
D. Pressure variation
Answer: A
9. Viscosity is a measure of:
A. Fluid's resistance to flow (internal friction)
B. Fluid density
C. Surface energy
D. Elasticity
Answer: A
10. SI unit of dynamic viscosity is:
A. Pa·s (N·s/m^2)
B. Pa
C. kg/m^3
D. m^2/s
Answer: A
11. For laminar flow in a circular pipe, volumetric flow rate Q follows Poiseuille's law and is proportional to:
A. r^2
B. r^4
C. r
D. 1/r
Answer: B
12. Reynolds number Re = ρ v D / μ predicts transition between:
A. Solid and fluid
B. Laminar and turbulent flow
C. Liquids and gases
D. Viscous and inviscid
Answer: B
13. Bernoulli's equation relates pressure, kinetic energy per unit volume and:
A. Potential energy per unit volume (ρ g h)
B. Viscous dissipation
C. Temperature
D. Entropy
Answer: A
14. Continuity equation for incompressible fluid states that A1 v1 = A2 v2. This expresses:
A. Conservation of mass
B. Conservation of energy
C. Conservation of momentum
D. Conservation of charge
Answer: A
15. Pitot tube measures:
A. Static pressure only
B. Total (stagnation) pressure and hence dynamic pressure
C. Viscosity
D. Surface tension
Answer: B
16. Surface tension has units of:
A. N/m
B. N·m
C. Pa
D. J
Answer: A
17. Capillary rise h in a tube of radius r is given by h = 2γ cosθ /(ρ g r). Here γ is:
A. Viscosity
B. Surface tension
C. Density
D. Pressure
Answer: B
18. Terminal velocity of a small sphere falling in viscous fluid (Stokes regime) is proportional to:
A. r
B. r^2
C. 1/r
D. Independent of r
Answer: B
19. Stokes' law for drag on a sphere in viscous fluid gives drag F = 6 π μ r v for:
A. Turbulent flow
B. Laminar (creeping) flow, low Re
C. Compressible flow
D. High speed flow
Answer: B
20. Viscous dissipation converts mechanical energy into:
A. Potential energy
B. Thermal energy (heat)
C. Kinetic energy only
D. Surface energy
Answer: B
21. For incompressible fluid, density changes with pressure are:
A. Significant
B. Negligible (approx. constant density)
C. Infinite
D. Random
Answer: B
22. Mach number is ratio of object speed to:
A. Speed of sound in the fluid
B. Kinematic viscosity
C. Flow velocity
D. Thermal speed
Answer: A
23. Flow is said to be steady if:
A. Velocity at a point changes with time
B. Velocity at a point does not change with time
C. Flow is turbulent
D. Density varies rapidly
Answer: B
24. For ideal (inviscid, incompressible) fluid, Bernoulli's equation is applicable along:
A. Any streamline
B. Across streamlines only
C. Not applicable
D. Only for turbulent flow
Answer: A
25. Dynamic pressure (½ ρ v^2) equals difference between:
A. Total and static pressure
B. Static and total pressure
C. Hydrostatic and atmospheric pressure
D. Viscous and inertial forces
Answer: A
26. A manometer measures pressure difference using column of:
A. Liquid of known density
B. Gas
C. Solid
D. Vacuum
Answer: A
27. For a floating body, centre of buoyancy is the centre of mass of:
A. The body
B. Displaced fluid (submerged volume)
C. Earth
D. None
Answer: B
28. Added mass effect arises when an accelerating body in fluid drags along:
A. Extra fluid inertia (virtual mass)
B. More buoyant force
C. Lower viscosity
D. Surface tension
Answer: A
29. Viscous boundary layer thickness δ grows with distance x along a flat plate roughly as:
A. δ ∝ x
B. δ ∝ √(x)
C. δ ∝ x^2
D. δ ∝ 1/x
Answer: B
30. Viscosity of liquids generally ______ with temperature, while viscosity of gases ______ with temperature.
A. Increases; decreases
B. Decreases; increases
C. Unchanged; unchanged
D. Both increase
Answer: B
31. Surface tension causes small droplets to be:
A. Cubic
B. Spherical (minimize surface area)
C. Cylindrical
D. Random shape
Answer: B
32. Lubrication between moving surfaces is achieved by:
A. Increasing friction
B. A thin viscous film that reduces direct contact
C. Removing fluid
D. Cooling only
Answer: B
33. Cavitation in liquids occurs when local pressure falls below:
A. Atmospheric pressure
B. Vapour pressure of the liquid
C. Zero
D. Hydrostatic pressure
Answer: B
34. The Hagen–Poiseuille equation assumes:
A. Turbulent flow in pipe
B. Laminar, steady, incompressible flow in circular pipe
C. Compressible flow
D. Unsteady flow only
Answer: B
35. For very low Reynolds number (Re << 1), flow regime is called:
A. Turbulent
B. Laminar (creeping/Stokes flow)
C. Supersonic
D. Transitional
Answer: B
36. Dimensional unit of kinematic viscosity ν is:
A. m^2/s
B. Pa·s
C. N/m^2
D. m/s
Answer: A
37. In a U-tube partially filled with same liquid, the liquid levels on both sides are:
A. Unequal due to gravity
B. Equal if open to same atmosphere
C. Depend on viscosity only
D. Infinite
Answer: B
38. For a submerged body, if centre of gravity lies above centre of buoyancy, the equilibrium is:
A. Stable
B. Unstable (tends to topple)
C. Neutral
D. Depends on velocity
Answer: B
39. Flow separation from a body surface commonly leads to:
A. Reduced drag
B. Increased pressure recovery
C. Increased drag and turbulence
D. Laminarisation
Answer: C
40. Sinking speed of a particle in a fluid is influenced by:
A. Particle size and density, fluid viscosity and density
B. Only particle color
C. Only atmospheric pressure
D. None
Answer: A
41. The pressure at a point in a static fluid is same in all directions (true or false)?
A. True
B. False
C. Only for gases
D. Only near surface
Answer: A
42. Boyle's law for gases (at constant temperature) relates pressure and volume as:
A. P ∝ V
B. P ∝ 1/V
C. P × V = constant
D. Both B and C
Answer: D
43. Bulk modulus K of a fluid relates pressure change to:
A. Shear strain
B. Volumetric strain (ΔV/V)
C. Linear strain
D. Surface tension
Answer: B
44. Compressibility of liquids is generally:
A. High
B. Very low (nearly incompressible)
C. Infinite
D. Same as gases
Answer: B
45. In the expression for Reynolds number, increasing characteristic length D tends to:
A. Decrease Re
B. Increase Re
C. No effect
D. Make flow steady
Answer: B
46. Wind blowing over water surface can generate surface waves by:
A. Viscous heating
B. Shear stress and pressure fluctuations transferring energy to surface
C. Changing density
D. Capillarity only
Answer: B
47. The concept of streamlines applies to:
A. Steady flow (streamlines coincide with flow direction)
B. Unsteady only
C. Static fluids
D. Random flows only
Answer: A
48. The drag coefficient for a sphere depends primarily on:
A. Reynolds number
B. Colour of sphere
C. Ambient pressure only
D. None
Answer: A
49. Hydrostatic paradox states that fluid pressure on a surface depends on:
A. Shape of container
B. Height of fluid column above the surface
C. Total volume only
D. Colour of fluid
Answer: B
50. Incompressible potential flow is characterized by velocity field that is:
A. Irrotational and divergence-free
B. Rotational only
C. Divergent only
D. Random
Answer: A