# Important Questions Class 11 Physics Chapter 10

## Important Questions Class 11 Physics Chapter 10

### Important Questions for CBSE Class 11 Physics Chapter 10 – Mechanical Properties of Fluids

CBSE Class 11 Physics Chapter 10 – Mechanical Properties of Fluids – Important Questions cover the common physical properties of various fluids. Furthermore, it specifies that fluids include both liquids and gases, rather than just liquids. Fluids are distinguished from solids by these properties.

Extramarks Important Questions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids is compiled in a concise manner by subject matter experts. These are useful for students who find it convenient to study in a question-answer format. These questions are taken from past years’ question papers and NCERT books so that students can learn how to form the answers and study the questions that are most likely to be asked.

### CBSE Class 11 Physics Chapter 10 Important Questions

Study Important Questions for Class 11 Physics Chapter 10 – Mechanical Properties of Fluids

Some of the Class 11 Physics Chapter 10 Important Questions are discussed below. Click on the link provided to review additional important questions.

### 1 Mark Answers and Questions

Q1. Describe the floatation law.

A1. According to the law of floatation, a body will float in a liquid if the weight of the liquid displaced by the immersed part of the body is equal to or greater than the weight of the body.

Q2. Is human blood pressure higher in the feet than in the brain?

A2. It is self-evident that the blood column in the human body is higher at the feet than at the brain. Because we know that pressure is directly proportional to the height of the liquid column, the pressure would be greater at the feet than at the brain.

### 2 Marks Answers and Questions

Q1. State the angle of contact and the values on which it is dependent.

A1. The angle of contact between a liquid and a solid can be defined as the angle enclosed between the tangents to the liquid surface and the solid surface inside the liquid, with both tangents drawn at the point of liquid-solid contact. The angle of contact is known to be affected by the following.

• Depending on the nature of the liquid and solid in contact
• The medium that exists above the liquid’s free surface

Q2. Despite the fact that pressure is force divided by area, and force is a vector, hydrostatic pressure is a scalar quantity. Explain.

A2. When a force is applied to a liquid, the pressure is distributed evenly throughout the liquid. Because the pressure has no fixed direction, we can classify it as a scalar quantity.

### 3 Marks Answers and Questions

Q1. Define terminal velocity and give it an expression.

A1. The maximum constant velocity attained by a body falling freely in a viscous medium is known as terminal velocity.

Three forces act on a small, spherical body as it falls freely through a viscous medium.

• Body weight acting vertically downwards
• Buoyancy-induced upthrust = weight of the liquid displaced
• Viscous drag (FV) acts in the opposite direction of body motion.

Let, ρ = Density of material

R = Radius of spherical body

So, ρ0 = Density of Medium.

∴True weight of the body = w = volume × density × g

W = 43πr3ρg

Upward thrust, FT

= Volume of Medium displaced

FT = 43πr3ρ0g

V = Terminal velocity of body

According to Stoke’s law,

FV = 6πηrv

When the body attains terminal velocity, then

FT+FV = W

⇒43πr3ρ0g+6πηrv = 43πr3ρg

V = 2r2(s−ρ0)g9η

1) V is directly proportional to the radius of the body and the pressure difference between the material and the medium.

2) V inversely depends on the coefficient of viscosity.

Therefore, terminal velocity can be described by the expression V = 2r2(s−ρ0)g9η.

### 4 Marks Answers and Questions

Q1. Explain why

1. a) The blood pressure in humans is greater at the feet than at the brain.

Ans: The pressure of a liquid with density ρ, with the liquid column of height h, is given by the relation:

P = ρhg

where g is the acceleration due to the gravity

It can be inferred that pressure is directly proportional to height. In the case of humans, the circulatory system can be considered as the liquid (blood)-column.

The height of the column is the least at the head level and the maximum at the feet; hence, the pressure (blood pressure) in human vessels depends on this height. Therefore, the blood pressure at the feet is higher than at the brain.

(b) Even though the atmosphere is more than 100 km high, the atmospheric pressure drops to nearly half its value at a height of about 6 km.

Ans: The density of a fluid is determined by how much fluid is pressed against it. This means that the density of the air is highest near sea level.

The density of air increases as one ascends from the sea surface. The total mass squeezes the layer of air over here to nearly half  the pressure value at sea level at a height of about 6 km.

When only minor changes in altitude occur, atmospheric pressure is proportional to density. When larger height scales are considered, the density itself depends on the height, and pressure is no longer linearly dependent. Furthermore, the pressure rises faster than the linear dependence expected for small changes in altitude.

(c) Despite the fact that pressure is force divided by area, hydrostatic pressure is a scalar quantity.

Ans: Pressure is defined mathematically as the perpendicular force per unit area. As a result, only the force component perpendicular to the surface or parallel to the area vector is used. This component’s direction and the direction of the area vector are the same.

As a result, there is no longer any direction involved.

In other words, when force is applied to a liquid, pressure is transferred in all directions. As a result, hydrostatic pressure is a scalar physical quantity with no fixed direction.

### 5 Marks Answers and Questions

Q1. Explain the following.

1. a) The angle of contact between mercury and glass is obtuse, whereas the angle of contact between water and glass is acute.

Ans: The angle of contact () is the angle formed by the surface inside the liquid and the tangent to the liquid surface at the point of contact.

Let Sla denote the interfacial tension at the liquid-air interface, Ssl the interfacial tension at the solid-liquid interface, and Ssa the interfacial tension at the solid-air interface. The surface forces between the three media must be in equilibrium at all points of contact, i.e.,

cosθ = Ssa−Ssl/Sla

The angle of contact () is obtuse in the case of a mercury drop on glass, Ssl>Ssa.

Ssl contact() is acute in the case of water on the glass.

(b) Water spreads out on a clean glass surface, whereas mercury forms drops on the same surface. (In other words, water wets glass while mercury does not.)

Ans: Mercury molecules have a strong attraction among themselves and a weak attraction toward solids. As a result, the angle of contact becomes obtuse, and the molecules become closer and tend to form a drop.

In the case of water, the ratio of attraction between water molecules is lower than the ratio of attraction between water molecules and glass.

As a result, the water molecules remain closer to the glass molecules, keeping the glass surface moist.

1. c) The surface tension of a liquid is independent of its area.

Ans: Surface tension is defined as the force acting per unit length at the interface of a liquid with another material.

This force is not affected by the area of the surface.

As a result, surface tension is independent of the area of the liquid’s surface.

1. d) Water-containing detergent should have small angles of contact.

Ans: Detergent water has small angles of contact () because the detergent molecules are sticky; in other words, they are strongly attracted to the water molecules and the solid with which it comes into contact.

As a result, the water molecules can get closer to the solid.

That is, as Ssl<Sla and the value of cos = SsaSslSla increases, the value decreases, making it acute or small.

1. e) In the absence of external forces, a drop of liquid is always spherical in shape.

Ans: Surface tension pulls the surface together as tightly as possible in the absence of any other external force or pressure. Surface tension will attempt to reduce the area as much as possible.

Beyond a certain point, the volume cannot be reduced, and the shape with the smallest surface area for the same volume is a sphere.

As a result, a droplet tends to become spherical.

### CBSE Class 11 Physics Important Questions

Class 11 Physics Chapter 10 Fluid Mechanical Properties explain the various physical properties of fluids. Fluids are defined as any substance that continually deforms under external force in the chapter. Liquids, gases, and plasma are all examples of fluids.

Chapter 10 of Class 11 Physics explains all aspects of surface tension. Surface tension in liquids is caused by the attraction of molecules on the surface and beneath the surface. Simply put, the molecules on the surface of a liquid are pushed inward by molecules on the inside. As a result, liquids tend to take the shape that covers the least amount of surface area.

Class 11 Physics Chapter 10 Important Questions, covers all these topics in the chapter in a question-and-answer format. Students can also use these for revision and exam preparation. These answers are written in a clear and concise manner so that students clearly understand the chapter’s concepts.

Q.1 The velocity of a small ball of mass 10 g and density 7.8g/cc becomes constant when dropped in a container filled with glycerine. If the density of glycerine is 1.3g/cc ,find the viscous force acting on the ball?

Marks:5

Ans

Here mass of the ball is 10g

Q.2 Explain why surface tension of the liquid is independent of the area of contact of the liquid surface.

Marks:2

Ans Surface tension of a liquid is the force acting per unit length on a line drawn tangentially to the liquid surface at rest. As the force is independent of the area of liquid surface, hence the surface tension is also independent of the area of the liquid surface.

Q.3 Read the assertion and reason carefully to mark the correct option out of the options given below.
Assertion: According to the hydrostatic paradox, when the same liquid is filled up to the same height in containers of different shape, at the particular height the same pressure is recoded for all three containers.
Reason: Liquid exerts pressure normally on the wall of containers.

A-Assertion is true but reason is false.

B-Assertion and reason both are false.

C-Both assertion and reason are true and the reason is the correct explanation of the assertion.

D-Both assertion and reason are true but reason is not the correct explanation of the assertion.

Marks:1

Ans Let us consider three containers A, B and C. In all containers, liquid exerts pressure on the wall of the container and according to Newton’s third law, wall will also exert reaction force on liquid.

If a container’s wall is inclined to the vertical, then wall exerts a reaction force which tries to increase or decrease the pressure of the liquid.

In container A, the reaction of a vertical column is acting upward to reduce the liquid pressure. In container B reaction of the wall is acting horizontally as a cross-section of the vessel is uniform. In the case of vessel C, the reaction of has component V acting downward to increase the liquid thrust and to overcome the effect to the small quantity of liquid.

Q.4 An aeroplane of mass 30000 kg and total wing area of 120 m2 is in a level flight at some height. The difference in pressure between the upper and lower surfaces of its wings in kilopascals is (g =10ms-2)

A-2.5

B-5.0

C-10.0

D-12.5

Marks:1

Ans

$In level flight of an aeroplane, mg = pA p = mg A = ( 3 × 1 0 4 ×10 ) 20 =2.5 kPa$

Q.5 Two water pipes P and Q having diameters of 2 × 10-2 m and 4 × 10-2 m respectively are joined in series with the main supply line of water. The velocity of water flowing in pipe P is

A-4 times that of Q

B-2 times that of Q

C-1/2 times that of Q

D-1/4 times that of Q

Marks:1

Ans

4 times that of Q

$Using theorem of continuity; ?D P 2 v P = ?D Q 2 v Q v P = [ D Q D P ] 2 V Q = [ 4×10 -2 m 2×10 -2 m ] 2 ×V Q = 4V Q$

## Please register to view this section

### 1. What is the Reynolds number?

The laminar or turbulent flow pattern through a pipe is identified by its Reynolds number. The Reynolds number has no dimensions. It is used to determine whether the flow pattern in a pipe is laminar or turbulent. The ratio of internal forces to viscous forces determines the Reynolds number. The flow through the pipe is said to be turbulent if the Reynolds number is high, and laminar if the Reynolds number is low.

### 2. What are the mechanical properties of solids?

The mechanical properties of solids aid in understanding properties such as strength and deformation resistance. It describes the strength with which an object can withstand stress. When stressed, clay, for example, is easily deformed. As a result, it is less resistant to deformation, but objects such as iron cannot be easily deformed. Only when heated does iron change shape. This means it is highly resistant to deformation.

### 3. What is fluid?

Fluids are substances that flow easily in response to even the slightest external force. Fluids include gases and liquids because they can flow easily and conform to the shape of the container. Fluids have no fixed shape and can take on the shape of a vessel. Fluids have two primary characteristics:

1. When fluids are at rest, they are hydrostatic.
2. When fluids are in motion, they are hydrodynamic.

### 4. What are the mechanical properties of fluids?

A fluid is a substance that can flow under the influence of an external force. Because they can flow, gases and liquids are classified as fluids. Hydrostatics is the study of the mechanical properties of fluids. When external pressure is applied to a fluid, it can change shape. Fluids have distinct physical properties that aid in understanding their behaviour when subjected to external forces.

### 5. What is Bernoulli’s Principle?

According to Bernoulli’s Principle, the density and pressure of fluid are inversely proportional to each other in an ideal state.

It should be noted that fluid does not necessarily mean liquid, fluids also include gases. According to the principle, a fluid moving quickly exerts more pressure than one moving slowly. This fluid-nature principle has served as the foundation for many practical applications. A great practical application of this principle is the design of aeroplane wings.