Chapter 9: Mechanical Properties of Fluids Assertion-Reason Questions | physics Class 11 CBSE

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 1

Logic Check

Assertion (A): Pressure is a scalar quantity even though it is defined as force per unit area. Reason (R): The force that appears in the definition of pressure is the component of force normal to the area, and no unique direction can be assigned to pressure itself.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): The blood pressure in humans is greater at the feet than at the brain. Reason (R): Pressure in a fluid increases with depth; the feet are at a greater depth below the heart than the brain.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Three vessels of different shapes connected at the bottom show the same water level when filled. Reason (R): Liquid pressure at any depth depends only on the height of the liquid column, not on the shape or volume of the container.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 4

Logic Check

Assertion (A): A hydraulic lift can support a car with a force much smaller than the car's weight. Reason (R): In a hydraulic system, the pressure is the same at the small and large pistons, but force is proportional to area.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): In steady fluid flow, two streamlines can intersect at a point. Reason (R): At the point of intersection, the fluid particle would have two different velocity directions, making the flow unsteady.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 6

Logic Check

Assertion (A): According to the equation of continuity, fluid flows faster through the narrow section of a pipe than through the wider section. Reason (R): The equation of continuity Av = constant is a consequence of conservation of mass for incompressible fluid flow.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): The pressure at the narrow section of a horizontal pipe is less than at the wider section. Reason (R): At the narrow section the fluid flows faster; by Bernoulli's equation higher speed corresponds to lower pressure at the same height.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): Bernoulli's equation holds perfectly for real water flowing through a pipe. Reason (R): Real fluids have viscosity which converts kinetic energy to heat, causing energy loss not accounted for by Bernoulli's equation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): The speed of efflux from a small hole in a large open tank at depth h is √(2gh). Reason (R): This result follows from Bernoulli's equation and is identical to the speed of a body in free fall from height h.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): An aircraft wing generates lift because the pressure above the wing is higher than below. Reason (R): Air flows faster over the curved upper surface of the wing than below, and by Bernoulli's principle higher speed means lower pressure.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 11

Logic Check

Assertion (A): The viscosity of liquids decreases with rise in temperature. Reason (R): At higher temperatures, molecules of a liquid have greater kinetic energy, which weakens intermolecular cohesive forces responsible for viscosity.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): The viscosity of gases increases with increase in temperature. Reason (R): In gases, higher temperature increases random molecular motion and the rate of momentum transfer between adjacent layers, increasing viscosity.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): Terminal velocity of a sphere falling through a viscous fluid is directly proportional to the square of the radius of the sphere. Reason (R): From Stokes' Law, the terminal velocity vt = 2a²(ρ−σ)g/(9η), which shows vt ∝ a².

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 14

Logic Check

Assertion (A): A raindrop reaches terminal velocity during its fall through the atmosphere. Reason (R): As the raindrop accelerates, the viscous drag force (proportional to velocity) increases until it equals the net downward force, after which acceleration is zero.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 15

Logic Check

Assertion (A): Surface tension is a property of a liquid alone. Reason (R): Surface tension arises at the interface between two substances and depends on both materials; it is not the property of a single substance.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Molecules at the surface of a liquid have higher potential energy than those in the interior. Reason (R): Surface molecules have fewer neighbours, resulting in fewer intermolecular attractive interactions and less negative potential energy.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 17

Logic Check

Assertion (A): Free liquid drops are spherical when the effects of gravity are negligible. Reason (R): A sphere has the maximum surface area for a given volume, and liquid drops tend to maximise their surface area.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): The excess pressure inside a soap bubble is twice that inside a liquid drop of the same radius and same surface tension. Reason (R): A soap bubble has two liquid-air interfaces, while a liquid drop has only one, doubling the surface energy contribution to excess pressure.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): Water rises in a capillary tube whereas mercury is depressed. Reason (R): The angle of contact for water with glass is acute while for mercury with glass it is obtuse, making cos θ positive for water and negative for mercury in the capillary rise formula.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 20

Logic Check

Assertion (A): Capillary rise is greater in a tube of smaller radius. Reason (R): The formula h = 2S cos θ/(ρga) shows that h is inversely proportional to the radius a of the tube.

Reasoning Options

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Chapter sections

Chapter 9: Mechanical Properties of Fluids | Assertion-Reason

Assertion (A): Pressure is a scalar quantity even though it is defined as force per unit area. Reason (R): The force that appears in the definition of pressure is the component of force normal to the area, and no unique direction can be assigned to pressure itself.

Assertion (A): The blood pressure in humans is greater at the feet than at the brain. Reason (R): Pressure in a fluid increases with depth; the feet are at a greater depth below the heart than the brain.

Assertion (A): Three vessels of different shapes connected at the bottom show the same water level when filled. Reason (R): Liquid pressure at any depth depends only on the height of the liquid column, not on the shape or volume of the container.

Assertion (A): A hydraulic lift can support a car with a force much smaller than the car's weight. Reason (R): In a hydraulic system, the pressure is the same at the small and large pistons, but force is proportional to area.

Assertion (A): In steady fluid flow, two streamlines can intersect at a point. Reason (R): At the point of intersection, the fluid particle would have two different velocity directions, making the flow unsteady.

Assertion (A): According to the equation of continuity, fluid flows faster through the narrow section of a pipe than through the wider section. Reason (R): The equation of continuity Av = constant is a consequence of conservation of mass for incompressible fluid flow.

Assertion (A): The pressure at the narrow section of a horizontal pipe is less than at the wider section. Reason (R): At the narrow section the fluid flows faster; by Bernoulli's equation higher speed corresponds to lower pressure at the same height.

Assertion (A): Bernoulli's equation holds perfectly for real water flowing through a pipe. Reason (R): Real fluids have viscosity which converts kinetic energy to heat, causing energy loss not accounted for by Bernoulli's equation.

Assertion (A): The speed of efflux from a small hole in a large open tank at depth h is √(2gh). Reason (R): This result follows from Bernoulli's equation and is identical to the speed of a body in free fall from height h.

Assertion (A): An aircraft wing generates lift because the pressure above the wing is higher than below. Reason (R): Air flows faster over the curved upper surface of the wing than below, and by Bernoulli's principle higher speed means lower pressure.

Assertion (A): The viscosity of liquids decreases with rise in temperature. Reason (R): At higher temperatures, molecules of a liquid have greater kinetic energy, which weakens intermolecular cohesive forces responsible for viscosity.

Assertion (A): The viscosity of gases increases with increase in temperature. Reason (R): In gases, higher temperature increases random molecular motion and the rate of momentum transfer between adjacent layers, increasing viscosity.

Assertion (A): Terminal velocity of a sphere falling through a viscous fluid is directly proportional to the square of the radius of the sphere. Reason (R): From Stokes' Law, the terminal velocity vt = 2a²(ρ−σ)g/(9η), which shows vt ∝ a².

Assertion (A): A raindrop reaches terminal velocity during its fall through the atmosphere. Reason (R): As the raindrop accelerates, the viscous drag force (proportional to velocity) increases until it equals the net downward force, after which acceleration is zero.

Assertion (A): Surface tension is a property of a liquid alone. Reason (R): Surface tension arises at the interface between two substances and depends on both materials; it is not the property of a single substance.

Assertion (A): Molecules at the surface of a liquid have higher potential energy than those in the interior. Reason (R): Surface molecules have fewer neighbours, resulting in fewer intermolecular attractive interactions and less negative potential energy.

Assertion (A): Free liquid drops are spherical when the effects of gravity are negligible. Reason (R): A sphere has the maximum surface area for a given volume, and liquid drops tend to maximise their surface area.

Assertion (A): The excess pressure inside a soap bubble is twice that inside a liquid drop of the same radius and same surface tension. Reason (R): A soap bubble has two liquid-air interfaces, while a liquid drop has only one, doubling the surface energy contribution to excess pressure.

Assertion (A): Water rises in a capillary tube whereas mercury is depressed. Reason (R): The angle of contact for water with glass is acute while for mercury with glass it is obtuse, making cos θ positive for water and negative for mercury in the capillary rise formula.

Assertion (A): Capillary rise is greater in a tube of smaller radius. Reason (R): The formula h = 2S cos θ/(ρga) shows that h is inversely proportional to the radius a of the tube.

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