Fluids are substances that can flow and change shape under very small shear stress. Liquids and gases are called fluids because they do not have a fixed shape like solids.
Important Questions Class 11 Physics Chapter 9 help students practise Mechanical Properties of Fluids through definitions, formulas, conceptual answers, numericals and exam-style questions.
This chapter explains how liquids and gases behave at rest and in motion. Students learn why pressure increases with depth, how hydraulic machines work, why aircraft wings get lift, and why small liquid drops become spherical.
The chapter covers pressure, Pascal’s law, pressure with depth, streamline flow, Bernoulli’s principle, Torricelli’s law, viscosity, Stokes’ law, surface tension, drops, bubbles and capillary rise. These questions help students revise formulas, concepts, applications and common exam patterns in one place.
Key Takeaways from Important Questions Class 11 Physics Chapter 9
| Area |
What Students Should Revise |
| Chapter Name |
Mechanical Properties of Fluids |
| Chapter Number |
Chapter 9 |
| Main Idea |
Behaviour of liquids and gases at rest and in motion |
| Core Topics |
Pressure, streamline flow, Bernoulli’s principle, viscosity, surface tension |
| Important Laws |
Pascal’s law, Stokes’ law, Torricelli’s law |
| Main Numericals |
Pressure at depth, hydraulic lift, continuity equation, Bernoulli equation, surface tension |
| High-Scoring Areas |
Bernoulli’s principle, viscosity, capillary rise and pressure questions |
| Common Mistake |
Treating pressure as a vector quantity |
| Best Practice |
Learn formulas with conditions, units and applications |
Mechanical Properties of Fluids Class 11 Important Questions: Formula Sheet
Students should revise formulas before solving mechanical properties of fluids class 11 important questions. Most numericals come from pressure, continuity equation, Bernoulli’s principle, viscosity and surface tension.
| Concept |
Formula |
| Density |
ρ = m/V |
| Pressure |
P = F/A |
| Pressure at depth |
P = Pa + ρgh |
| Gauge pressure |
Pg = ρgh |
| Pascal’s law in hydraulic machine |
F₁/A₁ = F₂/A₂ |
| Equation of continuity |
A₁v₁ = A₂v₂ |
| Bernoulli’s equation |
P + 1/2ρv² + ρgh = constant |
| Torricelli’s law |
v = √2gh |
| Stokes’ law |
F = 6πηrv |
| Terminal velocity |
vt = 2r²(ρ − σ)g/9η |
| Surface tension |
S = F/l |
| Surface tension for soap film |
S = F/2l |
| Excess pressure in liquid drop |
ΔP = 2S/r |
| Excess pressure in soap bubble |
ΔP = 4S/r |
| Capillary rise |
h = 2S cosθ/ρgr |
Mechanical Properties of Fluids Questions and Answers on Fluids
These mechanical properties of fluids questions and answers test the basic concept of fluids. Students should answer them before moving to numericals.
Q1. What are fluids?
Fluids are substances that can flow. Liquids and gases are fluids because they have no fixed shape.
Q2. Why are liquids and gases called fluids?
Liquids and gases are called fluids because they can flow and take the shape of their container. Gases also fill the complete volume of the container.
Q3. How are fluids different from solids?
Fluids offer very little resistance to shear stress. Solids resist change in shape much more strongly.
Q4. Why does a liquid have a fixed volume but no fixed shape?
A liquid has enough molecular attraction to keep volume nearly fixed. It still flows and takes the shape of the container.
Q5. Why are gases more compressible than liquids?
Gases have large intermolecular spaces. So, pressure can reduce their volume much more than liquids.
Pressure in Fluids Class 11 Questions
Pressure in fluids class 11 questions appear in both theory and numericals. Students should remember that pressure depends on normal force and area.
Q6. Define pressure.
Pressure is the normal force acting per unit area.
Formula:
P = F/A
Its SI unit is pascal.
Q7. Why is pressure a scalar quantity?
Pressure is scalar because it has no direction. The force used in the pressure formula is the normal component of force on the surface.
Q8. Why does a sharp needle pierce skin more easily than a blunt object?
A sharp needle has a very small contact area. For the same force, smaller area creates larger pressure.
Q9. What is the SI unit of pressure?
The SI unit of pressure is pascal. One pascal equals one newton per square metre.
Q10. State the dimensional formula of pressure.
The dimensional formula of pressure is [ML⁻¹T⁻²].
Class 11 Physics Chapter 9 Important Questions on Density and Relative Density
Density questions help students solve pressure and fluid motion numericals. Keep all units in SI form.
Q11. Define density.
Density is mass per unit volume.
Formula:
ρ = m/V
Its SI unit is kg m⁻³.
Q12. What is relative density?
Relative density is the ratio of the density of a substance to the density of water at 4°C. It has no unit.
Q13. Why is density important in fluid pressure questions?
Density decides how much pressure a fluid creates at a given depth. A denser fluid creates more pressure for the same depth.
Q14. A fluid has mass 2 kg and volume 0.002 m³. Find its density.
Density = m/V
Density = 2/0.002
Density = 1000 kg m⁻³
Answer: The density of the fluid is 1000 kg m⁻³.
Pascal Law Class 11 Questions
Pascal law class 11 questions usually connect theory with hydraulic machines. Students should understand pressure transmission before solving sums.
Q15. State Pascal’s law.
Pascal’s law states that pressure applied to an enclosed fluid gets transmitted equally and undiminished in all directions.
Q16. Why does Pascal’s law work in hydraulic machines?
Pascal’s law works because enclosed liquids transmit pressure equally. A small force on a small piston can create a large force on a larger piston.
Q17. Name two applications of Pascal’s law.
Hydraulic lift and hydraulic brakes work on Pascal’s law.
Q18. Why do hydraulic brakes apply equal braking effort on all wheels?
Brake fluid transmits pressure equally to all wheel cylinders. This creates equal braking effort on the wheels.
Q19. A hydraulic lift has piston areas A₁ and A₂. If A₂ is larger than A₁, why does output force increase?
The same pressure acts on both pistons. Since force equals pressure multiplied by area, the larger piston gives larger force.
Pressure Variation with Depth Class 11 Physics Questions
Pressure in a liquid increases with depth. This topic often appears in reason-based and numerical questions.
Q20. Write the formula for pressure at depth h in a liquid.
The formula is:
P = Pa + ρgh
Here, Pa is atmospheric pressure, ρ is density, g is acceleration due to gravity and h is depth.
Q21. What is gauge pressure?
Gauge pressure is the excess pressure above atmospheric pressure.
Formula:
Pg = ρgh
Q22. Why does pressure increase with depth in a liquid?
Pressure increases with depth because the liquid column above that point has more weight.
Q23. Why do submarine walls need high strength?
Submarine walls face large pressure at great depth. Water pressure increases with depth as ρgh.
Q24. What is hydrostatic paradox?
Hydrostatic paradox means liquid pressure at the bottom depends on height, density and gravity, not on container shape.
Mechanical Properties of Fluids Numericals Class 11
These mechanical properties of fluids numericals class 11 help students revise formulas with units.
Q25. A swimmer is 10 m below the surface of water. Find gauge pressure. Take ρ = 1000 kg m⁻³ and g = 10 m s⁻².
Gauge pressure:
Pg = ρgh
Pg = 1000 × 10 × 10
Pg = 1 × 10⁵ Pa
Answer: Gauge pressure is 1 × 10⁵ Pa.
Q26. Find absolute pressure at 10 m depth if atmospheric pressure is 1.01 × 10⁵ Pa.
Absolute pressure:
P = Pa + ρgh
P = 1.01 × 10⁵ + 1 × 10⁵
P = 2.01 × 10⁵ Pa
Answer: Absolute pressure is 2.01 × 10⁵ Pa.
Q27. A force of 20 N acts on area 0.01 m². Find pressure.
P = F/A
P = 20/0.01
P = 2000 Pa
Answer: Pressure is 2000 Pa.
Q28. A hydraulic machine has A₁ = 5 cm² and A₂ = 100 cm². If F₁ = 50 N, find F₂.
Using Pascal’s law:
F₁/A₁ = F₂/A₂
50/5 = F₂/100
F₂ = 1000 N
Answer: Output force is 1000 N.
Streamline Flow Class 11 Physics Questions
Fluid dynamics begins with smooth flow. Class 11 physics fluids important questions often ask the difference between steady flow and turbulent flow.
Q29. What is streamline flow?
Streamline flow is smooth flow in which every fluid particle passing a point has the same velocity with time.
Q30. Define streamline.
A streamline is a curve whose tangent at any point gives the direction of fluid velocity at that point.
Q31. Why can two streamlines never cross?
Two streamlines cannot cross because a fluid particle cannot have two velocities at the same point.
Q32. What is turbulent flow?
Turbulent flow is irregular flow in which velocity changes randomly with time and position.
Q33. What is the equation of continuity?
For steady flow of an incompressible fluid:
A₁v₁ = A₂v₂
It follows conservation of mass.
Q34. Why does water flow faster through a narrow pipe?
The equation of continuity says Av remains constant. If area decreases, speed increases.
Bernoulli Theorem Class 11 Questions
Bernoulli theorem class 11 questions are high-scoring because they test pressure, speed and height together.
Q35. State Bernoulli’s principle.
Bernoulli’s principle states that the sum of pressure energy, kinetic energy and potential energy per unit volume remains constant along a streamline.
Formula:
P + 1/2ρv² + ρgh = constant
Q36. What conditions are needed for Bernoulli’s principle?
Bernoulli’s principle applies to steady, incompressible and non-viscous fluid flow along a streamline.
Q37. Why does pressure decrease when fluid speed increases?
In horizontal flow, Bernoulli’s equation gives P + 1/2ρv² = constant. If speed increases, pressure decreases.
Q38. Why does a spinning cricket ball deviate from its path?
A spinning ball creates different air speeds on two sides. This creates pressure difference and dynamic lift, called the Magnus effect.
Q39. How does Bernoulli’s principle explain aircraft lift?
Air moves faster over the upper surface of a wing. Pressure becomes lower above the wing, so the higher pressure below creates lift.
Torricelli Law Class 11 Questions
Torricelli’s law comes from Bernoulli’s principle. It explains fluid outflow from a small hole.
Q40. What is Torricelli’s law?
Torricelli’s law states that speed of efflux from a hole at depth h is:
v = √2gh
Q41. What does efflux mean?
Efflux means outflow of liquid from a container through a small opening.
Q42. Why does Torricelli’s law look like the formula for free fall?
Both formulas depend on gravity and height. The liquid behaves like it has fallen through height h.
Q43. A tank has a hole 5 m below water surface. Find speed of efflux. Take g = 10 m s⁻².
v = √2gh
v = √(2 × 10 × 5)
v = √100
v = 10 m s⁻¹
Answer: Speed of efflux is 10 m s⁻¹.
Viscosity Class 11 Questions
Viscosity class 11 questions test internal friction in fluids. Students should connect viscosity with slow flow and terminal velocity.
Q44. What is viscosity?
Viscosity is the internal friction of a fluid. It opposes relative motion between fluid layers.
Q45. Why does honey flow slower than water?
Honey has higher viscosity than water. Its layers resist relative motion more strongly.
Q46. What is the SI unit of coefficient of viscosity?
The SI unit of coefficient of viscosity is poiseuille or Pa s.
Q47. What is the dimensional formula of viscosity?
The dimensional formula of viscosity is [ML⁻¹T⁻¹].
Q48. How does temperature affect viscosity?
Viscosity of liquids decreases with temperature. Viscosity of gases increases with temperature.
Stokes Law Class 11 Questions
Stokes law class 11 questions often connect viscous drag with terminal velocity. Students should know the formula and physical meaning.
Q49. State Stokes’ law.
Stokes’ law gives the viscous drag force on a small sphere moving through a fluid:
F = 6πηrv
Here, η is viscosity, r is radius and v is velocity.
Q50. What is terminal velocity?
Terminal velocity is the constant velocity reached by a body falling through a fluid when net force becomes zero.
Q51. Why does a raindrop reach terminal velocity?
As a raindrop falls, viscous drag increases with speed. Finally, drag and buoyant force balance weight.
Q52. Write the formula for terminal velocity.
The formula is:
vt = 2r²(ρ − σ)g/9η
Here, ρ is density of sphere and σ is density of fluid.
Q53. How does terminal velocity depend on radius?
Terminal velocity is directly proportional to the square of radius. A bigger sphere falls faster through the same fluid.
Surface Tension Class 11 Questions
Surface tension class 11 questions appear in concept, reason and numerical forms. This topic needs molecular understanding.
Q54. What is surface tension?
Surface tension is force per unit length acting at a liquid surface. It also equals surface energy per unit area.
Q55. Why do liquid drops become spherical?
A sphere has minimum surface area for a given volume. Surface tension makes small drops take a spherical shape.
Q56. Why does surface tension decrease with temperature?
Higher temperature increases molecular motion. This reduces cohesive effect at the surface.
Q57. Why does soap help water spread on cloth?
Soap reduces surface tension and angle of contact. Water can then spread and wet the cloth better.
Q58. Why do insects walk on water?
Surface tension makes the water surface behave like a stretched membrane. Small insects can stand on it without breaking the surface.
Angle of Contact Class 11 Questions
The angle of contact decides whether a liquid wets a solid or forms droplets.
Q59. What is angle of contact?
Angle of contact is the angle between the tangent to the liquid surface and the solid surface at the point of contact.
Q60. Why does water wet glass?
Water forms an acute angle of contact with clean glass. Adhesive forces between water and glass support wetting.
Q61. Why does mercury not wet glass?
Mercury forms an obtuse angle of contact with glass. Cohesive forces in mercury dominate over adhesive forces.
Q62. What happens when angle of contact is small?
A small angle of contact helps the liquid spread on the solid surface.
Drops, Bubbles and Capillary Rise Class 11 Questions
Capillary rise class 11 questions test surface tension formulas and applications. Students should separate drops, bubbles and capillary tubes carefully.
Q63. What is excess pressure inside a liquid drop?
Excess pressure inside a liquid drop is:
ΔP = 2S/r
Q64. What is excess pressure inside a soap bubble?
Excess pressure inside a soap bubble is:
ΔP = 4S/r
A soap bubble has two surfaces, so the formula has 4S/r.
Q65. What is capillary rise?
Capillary rise is the rise of a liquid in a narrow tube due to surface tension.
Q66. Write the formula for capillary rise.
The formula is:
h = 2S cosθ/ρgr
Q67. Why does water rise more in a thinner capillary tube?
Capillary rise is inversely proportional to radius. A thinner tube has a smaller radius, so water rises higher.
Mechanical Properties of Fluids Class 11 Extra Questions
These mechanical properties of fluids class 11 extra questions help students practise beyond direct definitions.
Q68. Why is pressure the same at all points on the same horizontal level in a liquid at rest?
If pressure differed at the same level, fluid would flow horizontally. Since the fluid stays at rest, pressure must be equal.
Q69. Why does a dam have thicker walls at the bottom?
Water pressure increases with depth. The bottom of the dam faces greater pressure.
Q70. Why does a fast jet of water come out when we partially close a tap with fingers?
The opening area decreases. By continuity equation, water speed increases through the smaller gap.
Q71. Why does Bernoulli’s equation fail in turbulent flow?
Bernoulli’s equation needs steady flow. In turbulent flow, pressure and velocity change randomly.
Q72. Why does Bernoulli’s equation fail for highly viscous fluids?
Viscosity causes energy loss as heat. Bernoulli’s equation assumes no energy loss due to internal friction.
Q73. Why does detergent reduce the angle of contact?
Detergent acts as a wetting agent. It lowers surface tension and helps water spread.
Q74. Why are small drops spherical but large drops become flattened?
Surface tension dominates in small drops. Gravity becomes more important in large drops and changes their shape.
Q75. Why does oil rise in a cotton wick?
Oil rises through narrow spaces in the wick due to capillary action.
Class 11 Physics Chapter 9 Questions and Answers: Solved Numericals
These class 11 physics chapter 9 questions and answers include exam-style calculations.
Important Questions Class 11 Physics Chapter 9: Numerical Practice
Practise these questions after revising pressure, Bernoulli’s equation, continuity equation and surface tension formulas.
Q76. A liquid drop has surface tension 0.072 N m⁻¹ and radius 2 mm. Find excess pressure.
For a liquid drop:
ΔP = 2S/r
ΔP = 2 × 0.072 / 0.002
ΔP = 72 Pa
Answer: Excess pressure is 72 Pa.
Q77. A soap bubble has radius 4 mm and surface tension 0.025 N m⁻¹. Find excess pressure.
For a soap bubble:
ΔP = 4S/r
ΔP = 4 × 0.025 / 0.004
ΔP = 25 Pa
Answer: Excess pressure is 25 Pa.
Q78. Water flows through a pipe of area 4 cm² with speed 2 m s⁻¹. Find speed when area becomes 1 cm².
Using continuity equation:
A₁v₁ = A₂v₂
4 × 2 = 1 × v₂
v₂ = 8 m s⁻¹
Answer: Speed becomes 8 m s⁻¹.
Q79. A fluid flows horizontally. Speed changes from 2 m s⁻¹ to 6 m s⁻¹. If density is 1000 kg m⁻³, find pressure drop.
For horizontal flow:
P₁ − P₂ = 1/2ρ(v₂² − v₁²)
P₁ − P₂ = 1/2 × 1000 × (36 − 4)
P₁ − P₂ = 500 × 32
P₁ − P₂ = 16000 Pa
Answer: Pressure drops by 16000 Pa.
Q80. A capillary tube has radius 0.5 mm. Water has surface tension 0.073 N m⁻¹. Find capillary rise if θ = 0°, ρ = 1000 kg m⁻³ and g = 9.8 m s⁻².
h = 2S cosθ/ρgr
h = 2 × 0.073 × 1 / (1000 × 9.8 × 0.0005)
h = 0.146 / 4.9
h = 0.0298 m
Answer: Capillary rise is 2.98 cm.
Common Mistakes in Class 11 Physics Fluids Important Questions
Students often lose marks because they use formulas without checking conditions.
Common mistakes include:
- Treating pressure as a vector
- Using Bernoulli’s equation for turbulent flow
- Forgetting atmospheric pressure in absolute pressure questions
- Confusing gauge pressure with absolute pressure
- Using soap bubble formula for a liquid drop
- Using diameter instead of radius in capillary rise questions
- Forgetting that viscosity causes energy loss
- Missing units in numerical answers
Pressure has no direction, even though force has direction. Bernoulli’s equation applies only to steady, incompressible and non-viscous flow.
For gauge pressure, use ρgh. For absolute pressure, use Pa + ρgh.
Quick Revision Notes for Mechanical Properties of Fluids Important Questions
Fluids include liquids and gases. They can flow and have no fixed shape.
Pressure is normal force per unit area. It is a scalar quantity.
Pressure in a liquid increases with depth. Gauge pressure equals ρgh.
Pascal’s law explains hydraulic lift and hydraulic brakes.
The equation of continuity says A₁v₁ = A₂v₂ for steady incompressible flow.
Bernoulli’s principle connects pressure, speed and height along a streamline.
Viscosity is internal friction in fluids. Stokes’ law gives viscous drag on a sphere.
Surface tension acts at liquid surfaces. It explains spherical drops, bubbles, wetting and capillary rise.
CBSE Class 11 Physics Chapter-Wise Important Questions