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Q9.18: NCERT Physics - Part 2 MECHANICAL PROPERTIES OF FLUIDS 9.18 Solution: Figure 9.21 (a) shows a thin liquid film supporting a small…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often assume surface tension forces are geometry‑independent and claim the same weight is supported simply because the liquid and temperature are unchanged. In reality the upward force is $F=2\gamma L$ (two surfaces) and depends on the total length $L$ of the film’s edge in contact with the weight; any change in that length or the number of surfaces alters the supported weight. Always identify the actual contact line length in each configuration before concluding.

Figure 9.21 (a) shows a thin liquid film supporting a small weight = 4.5×10−24.5 \times 10^{-2} N. What is the weight supported by a film of the same liquid at the same temperature in Fig. (b) and (c)? Explain your answer physically.

Step-by-Step Verified Solution

**Answer:** 4.5×10−24.5 \times 10^{-2} N in both cases. **Step-by-Step Explanation:** Surface tension depends only on the nature of the liquid and the temperature, not on the shape or orientation of the film. Since the liquid and temperature are the same, the surface tension is the same in all cases. The length of the film is also the same (40 cm), so the supported weight is the same: 4.5×10−24.5 \times 10^{-2} N [citation:7].

All Questions in this Chapter

Q9.1 Q9.2 Q9.3 Q9.4 Q9.5 Q9.6 Q9.7 Q9.8 Q9.9 Q9.10 Q9.11 Q9.12 Q9.13 Q9.14 Q9.15 Q9.16 Q9.17 Q9.18 Q9.19 Q9.20

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