An ideal fluid:
Try it first: pick an option, then read the explanation.
Correct answer: B. is both incompressible and non-viscous
Asked in SSC JE 2012
Detailed Explanation
Classical hydrodynamics defines an ideal fluid through two essential mechanical idealizations.
Why “is both incompressible and non-viscous” Is Correct
An ideal fluid must satisfy two conditions simultaneously: it is non-viscous (zero internal friction) and incompressible (constant density).
| Fluid type | Viscosity changes with rate of shear? | Examples |
|---|---|---|
| Ideal fluid | No (μ = 0) | Theoretical only |
| Newtonian fluid | No (μ = const) | Water, air, oil, mercury |
| Non-Newtonian | Yes | Blood, toothpaste, ketchup, paint |
| Ideal plastic (Bingham) | No, but needs yield stress τ₀ first | Fresh concrete, drilling mud |
Why the Other Options Are Wrong
- A. obeys Newton's law of viscosity: Newtonian fluids obey Newton's law; ideal fluids have zero viscosity and do not need a viscosity law.
- C. is non-viscous: Non-viscous alone is incomplete; an ideal fluid must also be incompressible.
- D. Frictionless and compressible: Ideal fluids are incompressible, not compressible.
Key Point
An ideal fluid is defined as both incompressible and non-viscous.
Key Concepts for Students
- Zero viscosity implies zero boundary layer thickness and zero skin friction.
- Incompressible implies speed of sound is theoretically infinite.
- Euler's equation of motion applies strictly to ideal fluids.
Practice more: this is question 77 in Properties of Fluid MCQs – Part 6. Try the whole set and check every answer instantly.