12 Most VSAQ’s of Mechanical Properties of Fluids Chapter in Inter 1st Year Physics (TS/AP)

2 Marks

VSAQ-1 : Why are drops and bubbles spherical?

Drops and bubbles are spherical due to surface tension, which attracts water molecules and compels them to form the most efficient shape with the least outside area, resulting in a spherical form.

VSAQ-2 : What is Magnus effect?

The Magnus effect refers to a force produced by the rotation or spinning of an object, in accordance with Newton’s third law of motion. Gustav Magnus discovered this phenomenon in 1852 when studying the curved trajectory of a cannonball. The Magnus effect is commonly observed in sports like baseball and in aviation. It occurs as a result of the interaction between the object’s spinning motion and the air around it, leading to a curving effect on the object’s path and altering its trajectory and movement.

VSAQ-3 : Define Viscosity. What are its units and dimensions?

Viscosity is the measurement of a fluid’s resistance to flow, representing the internal friction that hinders its motion. The SI unit for dynamic or absolute viscosity is Pascal seconds (Pa s) or N s/m^2. Viscosity’s dimensional formula is [ML^-1T^-1], indicating that it is expressed in units of force multiplied by time per unit area. Viscosity holds significant importance in determining the flow characteristics of fluids across a wide range of applications and industries.

VSAQ-4 : What are the principle behind the carburetor of an automobile?

The carburetor in an automobile functions based on Bernoulli’s theorem. It comprises a venture channel (nozzle) through which air flows at a high velocity, resulting in a region of reduced pressure at the narrow neck. This decrease in pressure leads to the suction of petrol into the chamber, facilitating the formation of the precise air-fuel mixture needed for combustion within the engine. The carburetor effectively regulates the flow of both air and fuel, ensuring the efficient and correct operation of the internal combustion engine in the automobile.

VSAQ-5 : What is angle of contact?

The angle of contact refers to the angle formed between the tangent to the surface of a liquid at the point of contact and the solid surface submerged within the liquid. It serves as an indicator of how effectively a liquid adheres to or spreads across a solid surface. Conventionally, the contact angle is measured through the liquid, specifically at the junction where the liquid-vapor interface meets the solid surface. This angle plays a pivotal role in determining whether the liquid will take the form of a droplet on the surface (high contact angle) or extend to create a thin film (low contact angle).

VSAQ-6 : Give the expression for the excess pressure in an air bubble inside the liquid.

The excess pressure (Pexcess​) inside an air bubble submerged in a liquid can be expressed using the Young-Laplace equation:

$$P_{\text{excess}} = \frac{2T}{R}$$


  • Pexcess​ is the excess pressure inside the bubble,
  • T is the surface tension of the liquid-air interface,
  • R is the radius of the bubble.

This equation relates the excess pressure within the bubble to the surface tension of the liquid and the radius of the bubble.

VSAQ-7 : Give the expression for the excess pressure in a liquid drop.

The excess pressure (Pexcess​) inside a liquid drop can be expressed using the Laplace’s law equation:

$$P_{\text{excess}} = \frac{2T}{R}$$


  • Pexcess​ is the excess pressure inside the drop,
  • T is the surface tension of the liquid-air interface,
  • R is the radius of the liquid drop.

Laplace’s law relates the excess pressure within a liquid drop to the surface tension of the liquid and the drop’s radius.

VSAQ-8 : Give the expression for the excess pressure in a Soap bubble in air.

The excess pressure (Pexcess​) inside a soap bubble in air can be expressed using Laplace’s law:

$$P_{\text{excess}} = \frac{2T}{R}$$


  • Pexcess​ is the excess pressure inside the soap bubble,
  • T is the surface tension of the soap film,
  • R is the radius of the soap bubble.

Laplace’s law relates the excess pressure within a soap bubble to the surface tension of the soap film and the bubble’s radius.

VSAQ-9 : When water flows through a pipe, which of the layers moves fastest and slowest? justify them.

In water flow through a pipe, the layer of water closest to the pipe’s surface moves the slowest, while the layer at the center of the pipe moves the fastest. This is due to the no-slip condition, where the fluid in direct contact with the pipe adheres to it, experiencing strong adhesive forces, resulting in slower movement. In contrast, the central layer experiences less resistance from the pipe’s surface, allowing it to move faster.

VSAQ-10 : What are water proofing agents and water wetting agents? What do they do?

  1. Laminar Flow in Pipes: Water flow within a pipe can be described as exhibiting laminar flow, a particular pattern of fluid motion.
  2. Velocity Distribution in Laminar Flow: In laminar flow, the velocity of water varies across different layers within the pipe. Understanding the velocity distribution is crucial to comprehend how different layers of water move within the pipe.
  3. No-Slip Condition and Friction: The no-slip condition plays a fundamental role in laminar flow. It refers to the fact that water molecules in direct contact with the pipe’s surface adhere to it, experiencing friction due to strong adhesive forces.
  4. Variation in Velocity: As we move from the pipe’s surface towards the center, the layers of water experience less friction and consequently, higher velocity.
  5. Velocity Peaks at the Center: The layer with the highest velocity is located at the exact center of the pipe.
  6. Boundary Layer with Lowest Velocity: Conversely, the layer with the lowest velocity is situated at the boundary between the water and the pipe.
  7. Parabolic Flow Profile: This variation in velocity across different layers leads to the formation of a distinctive parabolic flow profile within the laminar flow, illustrating the fluid’s behavior.

VSAQ-11 : Why water droplets wet the glass surface and does not wet lotus leaf?

Water droplets wet glass surfaces because the angle of contact between the droplets and the glass is less than 90 degrees. This causes the droplets to spread and form a thin film on the glass.

Conversely, water droplets do not wet lotus leaves because the angle of contact between water and the lotus leaf surface is greater than 90 degrees. As a result, the droplets maintain a spherical shape on the lotus leaf’s hydrophobic surface, preventing wetting and allowing them to roll off, keeping the leaf dry.

VSAQ-12 : Mention any two examples(or)applications that obey Bernoulli’s theorem and justify them.

  1. Airplane Wing Design: Bernoulli’s theorem is crucial in the design of airplane wings. The curved shape of an airplane wing, or airfoil, causes the air above it to move faster and have lower pressure than the air below it. This pressure difference generates lift, allowing airplanes to overcome gravity and achieve flight. It aligns with Bernoulli’s principle, where faster-moving air corresponds to lower pressure.
  2. Blood Flow in Arteries: Bernoulli’s theorem is also applicable to understanding blood flow in arteries. Blood flowing through narrow arteries accelerates, creating lower pressure according to Bernoulli’s principle. This principle helps in drawing blood back into the heart during diastole, aiding circulation. Arteries’ constriction and expansion rely on Bernoulli’s principle to maintain proper blood flow, ensuring efficient distribution of oxygen and nutrients to body tissues.