Static Fluid and Shear Stress

Static fluid refers to a fluid that is not in motion, i.e., the velocity of the fluid is zero. In such a fluid, the pressure is constant throughout the fluid, and there is no net external force acting on the fluid. Due to this, the fluid can resist any force applied to it, and the fluid particles remain in their initial position.

On the other hand, shear stress refers to the force per unit area acting on a fluid parallel to the surface. In other words, it is the force required to move one layer of fluid over another layer of fluid. Shear stress is a measure of the fluid's resistance to flow.

Shear Stress in Static Fluid

In the case of a static fluid, the fluid is not in motion, and there is no velocity gradient across its layers. This means that there is no force acting on one layer of fluid to move it over the other layer of fluid. Hence, there is no shear stress in a static fluid.

Therefore, the correct answer to the given question is option 'A' - Shear stress in static fluid is always zero.

Conclusion

Shear stress is a measure of the resistance of a fluid to flow. In a static fluid, there is no velocity gradient across the layers of the fluid, and hence, there is no force acting on the fluid to move it over the other layers. Therefore, there is no shear stress in a static fluid.