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Q9.17: NCERT Physics - Part 2 MECHANICAL PROPERTIES OF FLUIDS 9.17 Solution: A U-shaped wire is dipped in a soap solution, and removed. The…

⚠️ Common Exam Traps & Conceptual Pitfalls

When a soap film supports a weight, the tension acts along both free surfaces. The effective length in the force balance is $2l$, not $l$. Forgetting to double the length leads to a factor‑of‑two error in the surface tension calculation.

A U-shaped wire is dipped in a soap solution, and removed. The thin soap film formed between the wire and the light slider supports a weight of 1.5×10−21.5 \times 10^{-2} N (which includes the small weight of the slider). The length of the slider is 30 cm. What is the surface tension of the film?

Step-by-Step Verified Solution

**Answer:** 2.5×10−22.5 \times 10^{-2} N/m **Step-by-Step Explanation:** The soap film has two free surfaces, so the total length is 2l=2×0.30=0.602l = 2 \times 0.30 = 0.60 m. Surface tension:
S=W2l=1.5×10−20.60=2.5×10−2 N/mS = \frac{W}{2l} = \frac{1.5 \times 10^{-2}}{0.60} = 2.5 \times 10^{-2} \text{ N/m}
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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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