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Chapter 10 class 11 in physics is the mechanical properties of fluids. Students get to score higher scores in subjects like physics because the questions in competitive exams from the subject are based on basic concepts, formulas and problems related to the chapter. Theories are difficult to memorize and hence, there arises a chance of losing marks. In this chapter, our experts have tried to explain these concepts in detail so that students find it easy to understand and practice problems on the basis of understanding of the chapter.

In this chapter, students may learn in detail the mechanical properties of liquids and gases, pressure, streamline flow, viscosity, surface tension and so on.

As we all know, a fluid is anything that has no fixed shape. Both liquids and gases are referred to as fluids because they have the tendency to flow. Fluids can yield to slightest external pressures. The study of mechanical properties of fluids is called Hydrostatics. The volume of liquid or gas depends on the pressure acting on it. Since liquids have a fixed volume, the change in volume due to the change in external pressure is less. It is not the same in the case of gases, as they do not have a fixed volume.

Let’s study more about the concepts of mechanical properties of fluids

Fluids are liquids and gases with the property to flow in a certain direction on the application of external force. Two major topics are studied when we talk about the mechanical properties of fluids. They are- hydrodynamics and hydrostatics.

In physics, hydrodynamics is the study that concerns the forces acting on or exerted by fluids. It deals with the motion of fluids and the forces acting on solid bodies that are immersed in fluids. It also focuses on the motion relative to them. In short, it is the study of fluids in motion. Thus, it is a vast branch of science, which we will study more later.

In this chapter, we will focus more on Hydrostatics

This branch of physics is concerned with the fluids at rest.

Pascal made an observation that the pressure in a fluid that is at rest is the same at all points, provided they are at the same height. He also inferred that the pressure difference depends upon the vertical distance between the two points. Thus, the pressure difference applied to the fluid which is enclosed can be transmitted undiminished to every point of the fluid and the container vessel’s walls as well.

It can thus be noted that when an incompressible fluid is passing between every second in a pipe of non-uniform cross-section, the volume will be the same as the steady flow.

Bernoulli’s principle states that the total energy of the water always remains constant, therefore when the flow of water in a system increases, the pressure necessarily decreases. When water starts to flow in a hydraulic system the pressure drops and when the flow of water stops, the pressure rises again.

Therefore, in a hydraulic system, the total energy head is equal to the sum of three individual energy heads.

This can be expressed as follows-

**Total Head = [Elevation Head + Pressure Head + Velocity Head]**

**Where,**

Elevation head- is the pressure due to the elevation of the water

Pressure head- is the height of a column of water that a given hydrostatic pressure in a system could support

Velocity head- is the energy present due to the velocity of the water.

The amount of energy required to increase the surface of the liquid by unit area is defined as surface tension. It means it is the property of the surface of the liquid to resist force. Moreover, it is the force that holds the liquid molecules bound together. Therefore, surface tension is the amount of the extra energy which the molecules at the interface have when compared to the interior. Surface tension is denoted by the Greek letter ‘sigma’.

Viscosity is the measure of the resistance exerted by fluids to gradual deformation by shear or tensile stress. Thus, it can be considered as the fluid’s resistance to flow. When we say honey is thicker, milk is thinner, we intend to mean the viscosity of the liquid. Thus, the liquid that tends to flow less is more viscous.

It is measured in terms of a ratio of shearing stress to the velocity gradient in a fluid.

The equation to determine the viscosity of a fluid

When a sphere of radius a is dropped in a fluid of viscosity v, the viscosity is given by **η=2ga2(Δρ)9v**

**Where,**

∆ρ is the density difference between the fluid and the sphere

a is the radius of the sphere

g is the acceleration due to gravity

v is the velocity of the sphere

FAQ (Frequently Asked Questions)

1. What is Pascal’s Law?

Ans- Pascal made an observation that the pressure in a fluid that is at rest is the same at all points, provided they are at the same height. He also inferred that the pressure difference depends upon the vertical distance between the two points. Thus, the pressure difference applied on the fluid which is enclosed can be transmitted undiminished to every point of the fluid and the container vessel’s walls as well.

It can thus be noted that when an incompressible fluid is passing between every second in a pipe of non-uniform cross-section, the volume will be the same as the steady flow.

2. Explain Viscosity.

Ans- Viscosity is the measure of the resistance exerted by fluids to gradual deformation by shear or tensile stress. Thus, it can be considered as the fluid’s resistance to flow. When we say honey is thicker, milk is thinner, we intend to mean the viscosity of the liquid. Thus, the liquid that tends to flow less is more viscous.

It is measured in terms of a ratio of shearing stress to the velocity gradient in a fluid.