An ideal fluid is:
Try it first: pick an option, then read the explanation.
Correct answer: B. Frictionless and incompressible
Asked in SSC JE 2004
Detailed Explanation
Exam questions frequently test the classic definition of an ideal fluid across different phrasing variations.
Why “Frictionless and incompressible” Is Correct
An ideal fluid has zero viscosity (frictionless) and cannot be compressed (incompressible), producing no shear resistance under any motion.
| 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. One which obeys Newton's law of viscosity: A fluid obeying Newton's law is a Newtonian fluid, which is a real fluid.
- C. Very viscous: Ideal fluids have zero viscosity, not high viscosity.
- D. Frictionless and viscous: Frictionless and viscous is a self-contradictory statement.
Key Point
An ideal fluid is frictionless (zero viscosity) and incompressible.
Key Concepts for Students
- Ideal fluid has no surface tension, no viscosity, and infinite bulk modulus.
- Useful for estimating pressure distributions far from solid surfaces.
- Real fluids approach ideal fluid behavior in zones where velocity gradients are very small.
Practice more: this is question 79 in Properties of Fluid MCQs – Part 6. Try the whole set and check every answer instantly.