Consider following statements:
A. Surface tension arises due to extra energy of the molecules at the interior as compared to the molecules at the surface, of a liquid.
B. As the temperature of liquid rises, the coefficient of viscosity increases.
C. As the temperature of gas increases, the coefficient of viscosity increases.
D. The onset of turbulence is determined by Reynold's number.
E. In a steady flow two stream lines never intersect.
Choose the correct answer from the options given below:
A.A, D, E only$\text{A, D, E only}$
B.C, D, E only$\text{C, D, E only}$
C.B, C, D only$\text{B, C, D only}$
D.A, B, C only$\text{A, B, C only}$
Solution & Explanation
Core Logic
Let's audit each fluid mechanics assertion:
Statement A: Incorrect. Surface tension arises because surface molecules possess higher potential energy compared to interior bulk molecules due to net cohesive forces pulling inward.
Statement B: Incorrect. With rising temperatures, cohesive forces in liquids weaken, causing viscosity to decrease.
Statement C: Correct. In gases, viscosity is governed by molecular collisions. Higher temperature leads to increased thermal activity and momentum exchange, increasing viscosity.
Statement D: Correct. Critical velocity and turbulence parameters are completely defined by the dimensionless Reynolds number.
Statement E: Correct. If streamlines intersected, a fluid particle at that spatial node would have two distinct velocity directions, violating steady-state constraints.
Step 1: Final Conclusion
Statements C, D, and E are correct, leading to option (2).
Pattern Recognition
Contrast Liquid vs Gas viscosity trends under temperature increments: Liquids down (intermolecular bounds weaken), Gases up (random thermal diffusion and collision rates escalate).
Keywords:#Surface tension arises due to extra energy#JEE Main 2025 Morning Q3#Mechanical Properties of Fluids JEE Main 2025#Surface Tension and Viscosity JEE Main 2025
More Mechanical Properties of Fluids Previous-Year Questions
Water flows through a horizontal tube as shown in the figure. The difference in height between the water columns in vertical tubes is 5 cm and the area of cross-sections at A and B are 6 cm²$6\text{ cm}^{2}$ and 3 cm²$3\text{ cm}^{2}$ respectively. The rate of flow will be ____ cm³$\text{cm}^{3}$/s. (take g = 10 m/s²$\text{m/s}^{2}$)
The figure illustrates a Venturi meter setup with water flowing through a variable cross-section tube.
In a horizontal Venturi meter, substituting V₂ = V₁ (A₁/A₂)$V_2 = V_1 (A_1/A_2)$ directly into ρ g h = (1)/(2)ρ (V₂² - V₁²)$\rho g h = \frac{1}{2}\rho (V_2^2 - V_1^2)$ is the standard path. Working in CGS vs MKS units requires careful tracking; convert VA$V_A$ to cm/s before multiplying by AA$A_A$ in cm².
Chapter Mix
Class 11 Physics: Mechanical Properties of Fluids
Q38jee_main_2026_21_jan_eveningSurface Tension
Surface tension of two liquids (having same densities), T₁$T_{1}$ and T₂$T_{2}$, are measured using capillary rise method utilizing two tubes with inner radii of r₁$r_{1}$ and r₂$r_{2}$ where r₁ > r₂$r_{1} > r_{2}$. The measured liquid heights in these tubes are h₁$h_{1}$ and h₂$h_{2}$ respectively. [Ignore the weight of the liquid about the lowest point of miniscus]. The heights h₁$h_{1}$ and h₂$h_{2}$ and surface tensions T₁$T_{1}$ and T₂$T_{2}$ satisfy the relation :
A.h₁ < h₂ and T₁ = T₂$h_1 < h_2 \text{ and } T_1 = T_2$
B.h₁ = h₂ and T₁ = T₂$h_1 = h_2 \text{ and } T_1 = T_2$
C.h₁ > h₂ and T₁ = T₂$h_1 > h_2 \text{ and } T_1 = T_2$
D.h₁ > h₂ and T₁ < T₂$h_1 > h_2 \text{ and } T_1 < T_2$
Solution
Related Formula
h = (2T θ)/(ρ g r)$$h = \frac{2T\cos\theta}{\rho g r}$$
Core Logic
Since we are assessing surface tension T$T$ as a property of the liquids, the question intends to compare liquids that are identical (implying T₁ = T₂$T_1 = T_2$ and same density).
Capillary rise formula block for Q38 - JEE Main 2026 Evening
By Jurin's Law, for identical liquids, the height of capillary rise is inversely proportional to the radius of the tube:
h ∝ (1)/(r)$$h \propto \frac{1}{r}$$
Step 1: Final Conclusion
Given r₁ > r₂$r_1 > r_2$, the inverse proportionality directly implies that the liquid will rise less in the wider tube.
Therefore, h₁ < h₂$h_1 < h_2$ and naturally T₁ = T₂$T_1 = T_2$ for the intrinsic property.
Pattern Recognition
Wider tubes (r$r$ large) cause lower capillary rises (h$h$ small). h ∝ 1/r$h \propto 1/r$ is a foundational inverse relationship in capillary action.
Chapter Mix
Class 11 Physics: Mechanical Properties of Fluids
Q47jee_main_2026_21_jan_eveningTerminal Velocity
The terminal velocity of a metallic ball of radius 6 mm$6 \text{ mm}$ in a viscous fluid is 20 cm/s$20 \text{ cm/s}$. The terminal velocity of another ball of same material and having radius 3 mm$3 \text{ mm}$ in the same fluid will be ________ cm/s.
For balls of the same material falling through the same fluid, all terms in the terminal velocity equation except the radius r$r$ are constant.
Therefore, vT ∝ r²$v_T \propto r^2$.
Terminal velocity is directly proportional to the square of the radius. Halving the radius (6 → 3$6 \to 3$) will drop the terminal velocity by a factor of 1/4$1/4$.
Chapter Mix
Class 11 Physics: Mechanical Properties of Fluids
Q33jee_main_2026_22_january_morningPascal's Law and Surface Tension
Given below are two statements:
Statement I : Pressure of fluid is exerted only on a solid surface in contact as the fluid-pressure does not exist everywhere in a still fluid.
Statement II: Excess potential energy of the molecules on the surface of a liquid, when compared to interior, results in surface tension.
In the light of the above statements, choose the correct answer from the options given below:
A.Statement I is true but Statement II is false$\text{Statement I is true but Statement II is false}$
B.Both Statement I and Statement II are false$\text{Both Statement I and Statement II are false}$
C.Both Statement I and Statement II are true$\text{Both Statement I and Statement II are true}$
D.Statement I is false but Statement II is true$\text{Statement I is false but Statement II is true}$
According to Pascal's law, pressure exists at every point in a liquid at rest, not just at boundaries. Thus, Statement I is false.
For interior molecules, net cohesive forces are zero, whereas surface molecules possess excess potential energy leading to surface tension. Thus, Statement II is correct.
Pattern Recognition
Sees: Fluid pressure definition + surface tension origin.
Shortcut: Recall Pascal's law (pressure everywhere) and surface energy excess.
Check: Matches option (4). ✓
Chapter Mix
Class 11 Physics: Mechanical Properties of Fluids
Q34jee_main_2026_22_january_eveningCapillarity and Surface Tension
When a part of a straight capillary tube is placed vertically in a liquid, the liquid raises up to certain height h. If the inner radius of the capillary tube, density of the liquid and surface tension of the liquid decrease by 1 % each, then the height of the liquid in the tube will change by ____%.
Practice past-year questions one chapter at a time. Pick an exam → subject → chapter and get every PYQ for that topic — pulled together from all past papers — with the chapter's key formulas alongside.
