Understanding Forces on a Fluid at Rest

When a fluid is at rest, it means there is no relative motion between its different layers or parts. This state of a fluid is known as a static fluid. We need to consider which types of forces can act within or on the surface of such a fluid.

Let's examine the forces listed in the options:

• Viscous Force: Viscosity is a property of fluids that measures their resistance to flow. Viscous forces arise from the friction between different layers of a fluid that are moving relative to each other. If a fluid is completely at rest, there is no relative motion between its layers. Therefore, viscous forces are typically absent in a fluid that is at rest.
• Hydrostatic Force: This force is a result of the pressure exerted by a fluid due to gravity. Hydrostatic pressure exists at any point within a fluid at rest and increases with depth. This pressure acts perpendicular to any surface immersed in the fluid, creating a hydrostatic force. So, hydrostatic forces definitely act on a fluid at rest.
• Gravity: Gravity is a fundamental force that acts on any mass. It pulls the mass of the fluid downwards. The force of gravity on the fluid is responsible for creating hydrostatic pressure within the fluid when it is at rest. Thus, gravity acts on a fluid at rest.
• Surface Tension: Surface tension is a force that acts along the surface of a liquid. It is caused by the cohesive forces between the liquid molecules at the surface, which are attracted more strongly to each other than to the molecules in the air above. This force makes the surface of a liquid behave like a stretched elastic membrane. Surface tension is present on the free surface of a liquid even when the bulk of the fluid is at rest.

Based on the analysis, the only force among the options that requires relative motion between fluid layers is the viscous force. Since a fluid at rest has no such relative motion, viscous forces do not act on it in the static state.

Therefore, the force that does NOT act on a fluid which is at rest is the viscous force.