Pressure can be defined as the thrust experienced per unit area of the surface of a liquid at rest. In other words, force is applied perpendicular to the surface of an object per unit area over which that force is distributed. Thus, force acting upon a smaller area has a greater impact. This is another way in which the concept of pressure can be explained. Various units are used to express pressure. The standard unit of pressure is Pascal (Pa). Pressure is quantified as scalar. To measure the pressure exerted by a fluid, a piston is generally used .

Pressure is represented by the following formula:

P= F/A, where ‘P’ represents pressure, ‘F’ means force, and ‘A’ is the area.

The categorization of flow of liquids can be either streamline flow or turbulent flow. The flow of fluid is said to be steady if at all points, the velocity of all the particles in the fluid remains constant with time. Streamline flow is the term for the path that steady fluid follows. The speed of flowing fluids upon crossing a certain critical margin does not remain steady and thus become turbulent flow.

For a steady incompressible fluid, the equation of continuity is represented as below: 

A1v1 = A2v2

Here, ‘A’ stands for the area of a cross-section of fluid, and ‘v’ for the velocity of fluid particles at the same point. The principle of conservation of mass in incompressible fluids is the basis for the formation of this equation. It also indicates that the speed of flow of fluid is greater in a narrower pipe and the speed of flow is less inside a wider pipe.

Bernoulli’s principle is basically an expression that interrelates pressure changes, velocity and altitude changes. It states that, when an incompressible fluid is flowing in a steady manner, the collective summation of pressure energy per unit mass, kinetic energy per unit mass (which is related to change in velocity), and potential energy per unit mass (which is related to change in altitude) remains as a constant. Bernoulli's principle is not applicable for non-steady and turbulent flows, as their pressure and the velocities keep changing with time.

Most of the fluids are not ideal in nature, hence, offer some resistance to motion. This resistance to fluid motion is much similar to the friction when a solid moves on a surface. The friction provided by liquid is called viscosity. This exists when a relative motion exists between layers of the liquid. The SI unit of viscosity is Poiseiulle (Pl). Also, there are other units, which are N s m^-2 or Pa s. Thin liquids like water, alcohol, etc. are less viscous than thick ones like coal tar, blood, glycerin, etc. The viscosity of liquids decreases with rising temperature while it increases in the case of gases.

For a flow to become turbulent, the rate of flow must be large. When the flow rate is high, the flow no longer remains laminar or steady but becomes turbulent. In a turbulent flow, at any point, the velocity of fluids vary enormously and randomly. In a turbulent flow, certain circular motions termed eddies are also created. Turbulence is caused by an obstacle placed in the path of a fast-moving fluid.

Osborne Reynolds, observed that for viscous fluid flowing at low rates, turbulence is rare. He defined a number which is dimensionless. Its value provides a rough idea about whether the flow would eventually become turbulent. This particular number defined by him is called the Reynolds R_e. 

R_e = ρvd/η 

Here ρ represents the density of the fluid that is flowing with a speed v, d represents the dimension of the pipe through which the fluid is flowing, and η is the viscosity of the flowing fluid. As it is a dimensionless number, it remains constant in any system of units. When a flow is steady, streamlined, or laminar, the Re < 1000 and in case of turbulent flow, the Re > 2000.

It is the property of the liquid by the virtue of which the free surface of the liquid when at rest, tends to have minimum area and as such, it behaves as a membrane. In other words, surface tension is a property of the surface of a liquid that allows its resistance to an external force because of the cohesive nature of its molecules. 

1. What is Streamline Flow?

Streamline flow of fluid is a type of flow in which fluid travels in steady paths. In streamline flow, the velocity, pressure, and other physical properties remain constant at each point of time.

2. What is Bernoulli’s Principle?

Bernoulli’s principle is an expression that relates the pressure changes with velocity and height changes. It is not applicable for turbulent flows, because the pressure and the velocities do not remain constant with time in such flows.

3. Why is the NCERT Solution for Class 11 Physics Chapter 10 important?

The NCERT Solution for Class 11 Physics Chapter 10 is important because it covers all the term-wise previous years’ questions. Students need to understand the question pattern. Students benefit from it, as they can plan their study routine and be more efficiently prepared for exams. With the help of NCERT solutions, students can answer all the questions and perform well in the exams.

4. What are the key features of Mechanical Properties of Fluids NCERT Chapter 10?

The Mechanical Properties of Fluids NCERT Chapter 10 covers all the important topics that are crucial to understanding the nature of various solids, liquids, and gases. Some important concepts like that of pressure, viscosity, the surface tension of fluids and liquid, Bernoulli’s principle, and Reynolds number are discussed in the chapter. The mathematical formulas representing the same are also given in the chapter. Sums related to those formulas are also included as examples within the chapter.

5. What are the mechanical properties of solids?

The mechanical properties of solids include the following four properties:

• Elasticity: The ability of an object to regain its shape after the application of force is called elasticity. Example: Spring is an elastic object.
• Plasticity: The property of permanent deformation upon acting of force is called plasticity. Example: All plastic materials.
• Ductility: The property by which a solid can be beaten into sheets or converted into thin wires/sheets/plates. Example: Gold or Silver has high ductility.
• Strength: The ability to withstand applied force without any change in shape. Different solids have different strengths.