A thin liquid film formed between a U-shaped wire and a light slider supports a weight of 1.5 × 10⁻² N (see figure). The length of the slider is 30 cm and its weight negligible. The surface tension of the liquid film is (a) 0.0125 N m⁻¹ (b) 0.1 N m⁻¹ (c) 0.05 N m⁻¹ (d) 0.025 N m⁻¹

Answer: (d). The film has two surfaces, each pulling the slider with TL: total 2TL. Equilibrium: 2TL = W ⇒ T = W/2L. T = 1.5×10⁻²2 × 0.30 = 0.025 N m⁻¹. So the answer is (d).

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Force due to Surface Tension

A thin liquid film formed between a U-shaped wire and a light slider supports a weight of $1.5 \times 10^{-2}\text{ N}$ (see figure). The length of the slider is $30\text{ cm}$ and its weight negligible. The surface tension of the liquid film is

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Answer: (d)

The film has two surfaces, each pulling the slider with $TL$ : total $2TL$ .
Equilibrium: $2TL = W \Rightarrow T = \dfrac{W}{2L}$ .
$T = \dfrac{1.5\times10^{-2}}{2 \times 0.30} = 0.025\text{ N m}^{-1}$ . So the answer is (d).

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