In fluid mechanics, if there were no gravitational field acting on a fluid, which one of the following effects would no longer exist for the fluid?

Introduction / Context:
Fluids such as liquids and gases exhibit several important properties, including pressure, viscosity, surface tension, and buoyant force. Many of these are familiar from everyday experiences, such as floating objects, flowing water, and droplets forming on leaves. This question asks you to think conceptually about which effect would disappear if gravity were completely absent. It tests your understanding of which fluid properties fundamentally depend on the gravitational field and which are independent of it.

Given Data / Assumptions:

• We consider an idealised situation where gravitational acceleration is zero.
• The fluid still exists with molecules interacting through intermolecular forces.
• Properties listed include viscosity, surface tension, hydrostatic pressure due to weight, and upward buoyant thrust.
• We assume no other external fields create equivalent effects.

Concept / Approach:
Buoyant thrust or upthrust on a body immersed in a fluid is described by Archimedes principle. It states that the upward force equals the weight of the fluid displaced. Weight itself is the product of mass and gravitational acceleration. Without gravity, there would be no weight and hence no buoyant force supporting floating or rising bodies. In contrast, viscosity arises from internal friction between layers of fluid and depends on molecular interactions, not gravity. Surface tension arises due to cohesive forces at the liquid surface and also does not vanish without gravity. Pressure in a fluid has two components: microscopic kinetic pressure due to molecular motion, which would still exist, and hydrostatic pressure variation with depth due to weight, which would vanish, but the general concept of pressure would not disappear completely.

Step-by-Step Solution:
Step 1: Recall Archimedes principle: buoyant force equals the weight of displaced fluid.Step 2: Weight is given by mass multiplied by gravitational acceleration g.Step 3: If g becomes zero, the weight of any volume of fluid becomes zero, so the buoyant force also becomes zero.Step 4: Recognise that viscosity is caused by molecular friction and surface tension by cohesive forces, both of which do not require gravity.Step 5: Conclude that in the absence of gravity, upward buoyant thrust on immersed bodies would no longer be present, even though other properties remain.

Verification / Alternative check:
Consider astronauts aboard a spacecraft in orbit, where they experience effective weightlessness. Liquids behave strangely, forming spherical blobs that float rather than staying at the bottom of containers. Objects do not float or sink because there is no meaningful up or down direction defined by gravity, and the usual buoyant force relationships break down. However, the liquid still has viscosity, and its surface still behaves with surface tension, forming droplets. These examples from microgravity environments confirm that buoyant thrust is the effect that depends directly on gravity and disappears when gravity is absent.

Why Other Options Are Wrong:
Viscosity, option A, is an internal property of the fluid due to intermolecular forces and relative motion of layers; it exists even without gravity. Surface tension, option B, arises from cohesive forces at the surface, and droplets still form in space, so it does not vanish. Hydrostatic pressure due to weight, part of the idea behind option C, would lose its depth variation without gravity, but the general concept of pressure from molecular motion still exists. The question specifically points to an effect on a fluid that relies totally on gravity, which is the upward buoyant thrust on immersed bodies, covered by option D.

Common Pitfalls:
Many learners confuse pressure in general with pressure due to weight. While hydrostatic pressure gradients vanish without gravity, fluids still exert pressure through molecular collisions. Another pitfall is to think that all fluid properties disappear if gravity is removed, which is not true. Viscosity and surface tension are still very important in microgravity environments. To answer such questions correctly, always ask whether the definition of the property explicitly involves weight or gravitational acceleration. If it does, that property is likely to vanish when gravity is absent.

Final Answer:
Upward buoyant thrust on immersed bodies