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Q9.3: NCERT Physics - Part 2 MECHANICAL PROPERTIES OF FLUIDS 9.3 Solution: Fill in the blanks using the word(s) from the list appended…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently pick Bernoulli’s principle for the speed rise in a narrowed pipe, but the acceleration is actually dictated by the continuity equation $A_1v_1=A_2v_2$ (mass conservation). Use continuity first to find the velocity change, then apply Bernoulli only to relate the resulting pressure drop.
Fill in the blanks using the word(s) from the list appended with each statement: (a) Surface tension of liquids generally ... with temperatures (increases / decreases) (b) Viscosity of gases ... with temperature, whereas viscosity of liquids ... with temperature (increases / decreases) (c) For solids with elastic modulus of rigidity, the shearing force is proportional to ..., while for fluids it is proportional to ... (shear strain / rate of shear strain) (d) For a fluid in a steady flow, the increase in flow speed at a constriction follows (conservation of mass / Bernoulli's principle) (e) For the model of a plane in a wind tunnel, turbulence occurs at a ... speed for turbulence for an actual plane (greater / smaller)
Step-by-Step Verified Solution
**Answer:** (a) decreases (b) increases; decreases (c) shear strain; rate of shear strain (d) conservation of mass (e) greater
**Step-by-Step Explanation:**
(a) Surface tension of liquids decreases with increasing temperature because thermal agitation weakens intermolecular forces.
(b) Viscosity of gases increases with temperature (more molecular collisions), while viscosity of liquids decreases with temperature (molecules move more freely).
(c) For solids, shear stress is proportional to shear strain (Hooke's law). For fluids, shear stress is proportional to the rate of shear strain (Newton's law of viscosity) [citation:31].
(d) The increase in flow speed at a constriction is governed by the equation of continuity (conservation of mass), which states that the product of area and velocity is constant along a streamline [citation:31].
(e) In a wind tunnel model, turbulence occurs at a higher speed than for the actual plane because the model is smaller and Reynolds number matching requires higher speeds.
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