__________ stress can not be sustained by a fluid in equilibrium.

ANS:A - Shear

Shear stress is a type of stress that occurs when forces are applied parallel to a surface or plane within a material, causing different layers or particles of the material to slide past each other. This sliding motion leads to deformation of the material along the plane of applied force. Here's a more detailed explanation of shear stress:

1. Nature of Shear Stress: Shear stress arises from the relative motion or displacement of adjacent layers or particles within a material. It occurs when forces are applied parallel to a surface or plane, rather than perpendicular to it as in tensile or compressive stress.
2. Shearing Forces: Shearing forces act tangentially along the surface or plane of interest. These forces induce a tendency for adjacent layers or particles of the material to slide past each other in opposite directions, resulting in deformation.
3. Deformation Pattern: In response to shear stress, the material undergoes a distortion or shearing deformation along the plane of applied force. This deformation pattern is characterized by a change in shape or orientation of material elements, typically resulting in a shear strain.
4. Shear Stress Formula: Shear stress (τ) is calculated as the force applied parallel to the surface divided by the area over which the force is applied. Mathematically, it is represented as: τ=AF​ where:
   • τ = Shear stress (measured in units of force per unit area, such as Pascals or Newtons per square meter)
   • F = Applied force parallel to the surface
   • A = Area over which the force is applied
5. Applications: Shear stress is encountered in various engineering and scientific fields, including civil engineering, mechanical engineering, materials science, and fluid mechanics. It plays a crucial role in the behavior of materials under loading conditions, the design of structural components, and the analysis of fluid flow and deformation.