A real fluid in which the shear stress is directly proportional to the velocity gradient is known as:
Try it first: pick an option, then read the explanation.
Correct answer: B. Newtonian fluid
Asked in SSC JE 23.03.2021 (Morning Shift)
Detailed Explanation
The relationship between shear stress and velocity gradient defines the rheological character of a fluid. A constant slope indicates that viscosity is independent of shear rate.
Why “Newtonian fluid” Is Correct
Newtonian fluids strictly obey Newton's law of viscosity (\(\tau = \mu \frac{du}{dy}\)), exhibiting a constant coefficient of viscosity independent of the shear rate or agitation.
| 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. non-Newtonian fluid: Non-Newtonian fluids have variable apparent viscosity that changes with shear rate.
- C. ideal fluid: Ideal fluids have zero viscosity and therefore cannot support shear stress.
- D. plastic fluid: Plastic fluids require an initial threshold yield stress before continuous deformation begins.
Key Point
Newtonian fluids exhibit a linear relationship: τ = μ(du/dy) with constant μ.
Key Concepts for Students
- Common Newtonian fluids: water, air, light oils, gasoline, alcohol.
- The slope of the τ versus du/dy curve represents dynamic viscosity μ.
- Newtonian behavior holds across wide ranges of shear rates for simple molecular liquids.
Practice more: this is question 26 in Properties of Fluid MCQs – Part 2. Try the whole set and check every answer instantly.