Mechanical Properties In Liquids Physics NCERT Textbook PDF

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NCERT Class 11 Physics Textbook Chapter 10 With Answer PDF Free Download

Mechanical Properties in Liquids

Chapter 10: Mechanical Properties in Liquids

10.1 Introduction

In this chapter, we shall study some common physical properties of liquids and gases. Liquids and gases can flow and are, therefore, called fluids.

It is this property that distinguishes liquids and gases from solids in a basic way.

Fluids are everywhere around us. Earth has an envelope of air and two-thirds of its surface is covered with water.

Water is not only necessary for our existence; every mammalian body constitutes mostly water. All the processes occurring in living beings including plants are mediated by fluids.

Thus understanding the behavior and properties of fluids is important. How are fluids different from solids? What is common in liquids and gases?

Unlike a solid, a fluid has no definite shape of its own. Solids and liquids have a fixed volume, whereas a gas fills the entire volume of its container.

We have learned in the previous chapter that the volume of solids can be changed by stress. The volume of solid, liquid, or gas depends on the stress or pressure acting on it.

When we talk about the fixed volume of solid or liquid, we mean its volume under atmospheric pressure.

The difference between gases and solids or liquids is that for solids or liquids the change in volume due to a change of external pressure is rather small.

In other words, solids and liquids have much lower compressibility as compared to gases.

Shear stress can change the shape of a solid keeping its volume fixed.

The key property of fluids is that they offer very little resistance to shear stress; their shape changes by application of very small shear stress. The shearing stress of fluids is about a million times smaller than that of solids.

10.2 Pressure

A sharp needle when pressed against our skin pierces it. Our skin, however, remains intact when a blunt object with a wider contact area (say the back of a spoon) is pressed against it with the same force.

If an elephant were to step on a man’s chest, his ribs would crack. A circus performer across whose chest a large, light but strong wooden plank is placed first, is saved from this accident.

Such everyday experiences convince us that both the force and its coverage area are important.

The smaller the area on which the force acts, the greater the impact. This concept is known as pressure.

When an object is submerged in a fluid at rest, the fluid exerts a force on its surface. This force is always normal to the object’s surface.

This is so because if there were a component of force parallel to the surface, the object will also exert a force on the fluid parallel to it; as a consequence of Newton’s third law.

This force will cause the fluid to flow parallel to the surface. Since the fluid is at rest, this cannot happen.

Hence, the force exerted by the fluid at rest has to be perpendicular to the surface in contact with it.

Language English
No. of Pages28
PDF Size2.1 MB

NCERT Solutions Class 11 Physics Chapter 10 Mechanical Properties in Liquids

Q.1: Explain why
(a) The blood pressure in humans is greater at the feet than at the brain
(b) Atmospheric pressure at a height of about 6 km decreases to nearly half of its value at the sea level, though the height of the atmosphere is more than 100 km
(c) Hydrostatic pressure is a scalar quantity even though the pressure is force divided by area


(i). The blood column to the feet is at a greater height than the head, thus the blood pressure in the feet is greater than that in the brain.

(ii). The density of the atmosphere does not decrease linearly with the increase in altitude, in fact, most of the air molecules are close to the surface. Thus, there is this nonlinear variation of atmospheric pressure.

(iii). In hydrostatic pressure the force is transmitted equally in all directions in the liquid, thus there is no fixed direction of pressure making it a scalar quantity.

Q.3: Fill in the blanks using the word(s) from the list appended with each statement:
(a) The surface tension of liquids generally … with temperatures (increases/decreases)
(b) The viscosity of gases … with temperature, whereas the viscosity of liquids … with temperature (increases/decreases)
(c) For solids with an elastic modulus of rigidity, the shearing force is proportional to …, while for fluids, it is proportional to … (shear strain/rate of shear strain)
(d) For a fluid in a steady flow, the increase in flow speed at a constriction follows (conservation of mass / Bernoulli’s principle)
(e) For the model of a plane in a wind tunnel, turbulence occurs at a … speed for turbulence for an actual plane (greater / smaller)


(a) Decreases

(b) increases; decreases

(c) shear strain; rate of shear strain

(d) conservation of mass; Bernoulli’s principle

(e) greater.

Q.4: Explain why
(a) To keep a piece of paper horizontal, you should blow over, not under, it
(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers
(c) The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection
(d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel
(e) A spinning cricket ball in the air does not follow a parabolic trajectory


(a) If we blow over a piece of paper, the velocity of air above the paper becomes more than that below it. As K.E. of air above the paper increases, in accordance with Bernoulli’s theorem its pressure energy and hence its pressure decreases.

Due to the greater value of pressure below the piece of paper = atmospheric pressure, it remains horizontal and does not fall.

(b) As per the equation of continuity area × velocity = constant. When we try to close a water tap with our fingers, the area of cross-section of the outlet of the water jet is reduced considerably as the openings between our fingers provide constriction  (regions of smaller area)
Thus, the velocity of the water increases greatly and fast jets of water come through the openings between our fingers.

(c) The size of the needle controls the velocity of flow and the thumb pressure controls pressure.  According to the Bernoulli’s theorem  P + \frac{ 1 }{ 2 } pv^{ 2 }21​pv2 = Constant

In this equation, the pressure P occurs with a single power whereas the velocity occurs with a square power. Therefore, the velocity has more effective compared to the pressure. It is for this reason that the needle of the syringe controls the flow rate better than the thumb pressure exerted by the doctor.

(d) This is because of the principle of conservation of momentum. While the flowing fluid carries forward momentum, the vessel gets a backward momentum.
(e) A spinning cricket ball would have followed a parabolic trajectory had there been no air. But because of air, the Magnus effect takes place. Due to the Magnus effect, the spinning cricket ball deviates from its parabolic trajectory.

Mechanical Properties in Liquids Textbook With Solutions PDF Free Download

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