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Question 1: What is fluid? Discuss briefly the viscosity of fluid and explain how the flow of a fluid is characterized by viscosity?



A substance that can flow is called fluid. Liquids and gases are combine called fluid as they have the property of flowing.


Viscosity is the internal friction between different layers of the fluid. It is defined as the property of the flowing fluid due to which it opposes the relative motion of different layers of the fluid.

Low viscosity fluid flows easily than the fluid which has high viscosity. As an example, water is less viscous than honey and flows more easily.

The opposing force between different layers is directly proportional to surface area A and the relative velocity v of the layers. It is inversely proportional to the distance x between them. Therefore,

Here η is the constant called the coefficient of viscosity.

For example, coefficient of viscosity for air is 0.019×103 N s m-2. Its value depends upon the nature of the fluid and temperature of the fluid. In case of liquids, when the temperature is raised the molecules vibrate more vigorously and the forces between them become weaker. Therefore, the viscosity of the liquid is decreased. In case the fluid is a gas, by increasing the temperature the velocities of the molecules increases. This increases the distances between the molecules and the viscosity of the gas decreases.)

Viscosity is the key characteristic that determines the rate of flow of a fluid. In a more viscous fluid, the layers of the fluid offer more resistance to the flow of one another and will flow slowly as compared to a less viscous fluid, where the resistance offered is less, under same conditions. As an example, honey is more viscous than water and flow slowly as compared to water.

The numerical value of resistance of the flow of a fluid between its different layers is called coefficient of viscosity. More viscous fluids have high value of the coefficient of viscosity.

Fluid Mechanics is the “study of fluids and that how forces affect them”. It has two major branches; fluid statics and fluid dynamics. The first studies fluid at rest and the second is concerned with the motion of fluids.
Fluid Dynamics play important role in our daily life. Fluid dynamics encompasses studies from ocean currents (samandari chapi) to blood circulation, from weather patterns to the studies of the evolution of stars and from wind turbines to oil pipelines.

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