Question

Which of the following forces does NOT act on fluid which is at rest?
1. Viscous
2. Hydrostatic
3. Gravity
4. Surface tension

Answer 1

Correct Answer - Option 1 : Viscous

Explanation:

Static fluid

• Static fluid, fluid at rest has no shear stress.
• Consequently, any force developed is only due to normal stresses i.e., pressure.
• The fluid is to be at rest if there is no relative movement between the adjacent particles of the fluid i.e. no shear stress is acting on the fluid.
• Hence, the viscous force is inconsiderable in this case.
• However, fluid particles are always acted upon by external pressure and their own self-weight.
• Surface tension acts only for fluid at rest condition. 

Viscous Force:

• It is equal to the product of shear stress (τ) due to the viscosity and surface area of the flow.
• It is present in fluid flow problems where viscosity is having an important role to play.