Mechanical Properties of Fluids — Class 11 MCQs with Answers

Mechanical Properties of Fluids — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   SI unit of pressure:

   AJouleBPascalCNewtonDWatt
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   Answer: Pascal

   Explanation: Pressure's SI unit is the pascal: 1 Pa = 1 N/m², force per area. Joule is energy and Watt is power — a common mix-up, but pressure is neither.

2. Q2 of 6Easy

   Pressure at depth h in fluid (density ρ):

   Ag hBρ hCρ g hDρ g
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   Answer: ρ g h

   Explanation: Hydrostatic pressure at depth h in a fluid of density ρ is P = ρgh, combining density, gravity, and depth — dropping g (leaving just ρh) is the frequent error since g is rarely written explicitly.

3. Q3 of 6Medium

   Hydraulic lift: small piston area 1 cm², force 10 N. Large piston area 100 cm². Force lifted:

   

   A10 NB1000 NC100 NDCannot determine
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   Answer: 1000 N

   Explanation: Pascal's law keeps pressure equal on both pistons: F₁/A₁ = F₂/A₂, so F₂ = 10 N × (100 cm²/1 cm²) = 1000 N. Using the area ratio the wrong way round gives the tempting but wrong 100 N.

4. Q4 of 6Medium

   Body of volume 100 cm³ in water (ρ = 1000). Buoyant force (g = 10):

   A100 NB10 NC1 NDCannot determine
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   Answer: 1 N

   Explanation: Buoyant force = ρ_water·V·g = 1000 kg/m³ × 100×10⁻⁶ m³ × 10 m/s² = 1 N. Forgetting to convert 100 cm³ to 10⁻⁴ m³ before multiplying is the common slip giving 10 N or 100 N instead.

5. Q5 of 6Hard

   Two pipes of lengths L and 2L have the same radius and carry the same liquid at the same volume flow rate. Ratio of the pressure differences across them (ΔP₁ : ΔP₂):

   A1:2B1:1C2:1D4:1
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   Answer: 1:2

   Explanation: Poiseuille's law gives Q = πPr⁴/(8ηL); for the same Q and radius, P is proportional to length L, so with lengths L and 2L the pressure ratio is P₁:P₂ = L:2L = 1:2 — inverting this ratio is the common mistake.

6. Q6 of 6Hard

   For sphere falling in viscous fluid with terminal velocity, all forces balanced:

   ANo forcesBJust gravityCDrag onlyDGravity = Buoyancy + drag
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   Answer: Gravity = Buoyancy + drag

   Explanation: At terminal velocity net force is zero, so downward weight is exactly balanced by the sum of buoyancy and Stokes' viscous drag: mg = F_buoyancy + F_drag — ignoring buoyancy and balancing weight against drag alone is the common error.

Mechanical Properties of Fluids notes: key concepts (Class 11 Physics)

Fluids at rest and in motion: pressure and how it varies with depth, Pascal's law and the hydraulic lift, Archimedes' principle and flotation, the continuity equation and Bernoulli's principle for streamline flow, viscosity with Stokes' law and terminal velocity, the Reynolds number, and surface tension with capillary rise.

Pressure in a fluid

The normal force per unit area exerted by a fluid. Pressure is a scalar and at any point in a fluid at rest it acts equally in every direction.

Pressure with depth

Pressure increases linearly with depth as ρgh, and depends only on depth, density and g — never on the shape or total volume of the container.

Gauge and absolute pressure

Gauge pressure is the excess over atmospheric pressure, which is what most instruments read. Absolute pressure adds atmospheric pressure back in.

Pascal's law

Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and to the walls of the container.

Hydraulic lift

A direct application of Pascal's law: a small force on a small piston produces a large force on a large one, in proportion to the ratio of areas.

Archimedes' principle

A body immersed in a fluid experiences an upward buoyant force equal to the WEIGHT of the fluid it displaces, not the weight of the body.

Flotation

A body floats when its average density is less than the fluid's, settling so the weight of displaced fluid exactly equals its own weight.

Streamline and turbulent flow

In streamline flow every particle passing a point follows the same path; above a critical speed the flow becomes chaotic and turbulent.

Equation of continuity

For an incompressible fluid the product of area and speed is constant along a tube of flow — this is simply conservation of mass, and it is why a narrowed pipe speeds the flow up.

Bernoulli's principle

Along a streamline, pressure plus kinetic energy per unit volume plus potential energy per unit volume stays constant. It is energy conservation for an ideal fluid.

Applications of Bernoulli

Explains aerofoil lift, the Venturi meter, the atomiser and the spin of a swinging cricket ball — everywhere faster flow accompanies lower pressure.

Viscosity

Internal friction between adjacent layers of a fluid moving at different speeds. Liquids become less viscous when heated, while gases become more so.

Stokes' law

The viscous drag on a small sphere moving slowly through a fluid, 6πηrv — proportional to speed, radius and the coefficient of viscosity.

Terminal velocity

The constant speed reached when drag plus buoyancy balance weight, so the net force is zero. It varies as the square of the radius.

Reynolds number

A dimensionless ratio of inertial to viscous forces that predicts the flow regime — below about 1000 flow is laminar, above about 2000 it turns turbulent.

Surface tension and capillarity

The force per unit length in a liquid surface, arising from unbalanced molecular attraction, which drives capillary rise and makes small drops spherical.