Forces acting in a moving fluid: Which characteristic forces are present simultaneously in a moving liquid under real (practical) flow conditions?

Introduction / Context:
Any real liquid in motion experiences several fundamental forces at once. These include inertia (due to mass and acceleration), viscous (due to internal friction), gravity (weight), pressure, and sometimes surface tension and elastic effects. The question asks you to recognize that multiple forces act together rather than in isolation.

Given Data / Assumptions:

• Continuum (fluid) mechanics framework.
• Real liquid with nonzero viscosity.
• Flow may be internal (pipes) or external (around bodies).

Concept / Approach:
Force balances in fluids are captured by the Navier–Stokes equations. The terms in these equations correspond to inertia, pressure, viscous, and body forces (e.g., gravity). In typical engineering flows, at least inertia, viscosity, and gravity are simultaneously present, though one may dominate depending on the regime (e.g., high Reynolds number → inertia dominance).

Step-by-Step Solution:

Verification / Alternative check:
Dimensionless groups confirm coexistence: Reynolds (inertia/viscous), Froude (inertia/gravity), Euler (pressure/inertia), Weber (inertia/surface tension). These ratios only make sense because each force is present to some degree.

Why Other Options Are Wrong:

• Inertia force alone: Ignores friction and weight.
• Viscous force alone: Would imply a Stokes-like limit with no inertia/gravity—rare in general practice.
• Gravity force alone: Omits dynamics and frictional losses.

Common Pitfalls:
Thinking a single force “causes” the flow while others vanish. In reality, dominance varies by regime, but the other forces generally remain nonzero.

Final Answer:
all of these