Definition of Fluid: What is Shear Stress?

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SUMMARY

A fluid is defined as a substance that continually deforms under applied shear stress, as stated in the Wikipedia definition. This definition encompasses all liquids and gases, emphasizing that shear stress, which represents tangential pressure, is the primary factor causing flow. While fluids can also deform under perpendicular forces, such as in compressible gases, it is the continuous nature of shear stress that facilitates fluid movement. Therefore, shear stress is essential in understanding fluid dynamics, as it is the only force that induces flow in fluids.

PREREQUISITES
  • Understanding of shear stress and its role in fluid mechanics
  • Basic knowledge of fluid properties, including compressibility
  • Familiarity with the distinction between liquids and gases
  • Concept of normal stress versus shear stress in physics
NEXT STEPS
  • Research the principles of fluid dynamics using "Fundamentals of Fluid Mechanics" by Munson et al.
  • Explore the concept of shear stress in detail with "Mechanics of Fluids" by Merle C. Potter
  • Learn about the behavior of compressible fluids through resources on gas dynamics
  • Study the mathematical modeling of fluid flow, focusing on Navier-Stokes equations
USEFUL FOR

Students of physics, engineers specializing in fluid mechanics, and professionals involved in the study of fluid dynamics will benefit from this discussion.

Micko
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Hello,
I have found the following definition in wikipedia:
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress. All liquids and all gases are fluids.
However, it's pretty hard for me to understand this definition. Shear stress represents tangential pressure applied. Fluid will also deform under perpendicular force. Why tangential force is used in definition?

Thanks
 
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A fluid will deform by it's own weight, which can be tangential to the surface holding the fluid.
Not sure if that answers your question though.
 
Micko said:
Fluid will also deform under perpendicular force. Why tangential force is used in definition?
Gasses will deform under perpendicular force, ie they are compressible. Liquids do not. Fluids include both liquids and gasses, so compressibility cannot be part of the definition.

Also, even in a compressible fluid, the "perpendicular" deformation is not continuous. So a normal stress does not cause a fluid to flow, only a shear stress.
 

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