A) \[\text{3}\times \text{1}{{0}^{\text{-3}}}\text{N}{{\text{m}}^{\text{-1}}}\]
B) \[\text{2}\times \text{1}{{0}^{\text{-5}}}\text{N}{{\text{m}}^{\text{-1}}}\]
C) \[4\times \text{1}{{0}^{\text{-4}}}\text{N}{{\text{m}}^{\text{-1}}}\]
D) \[2.5\times \text{1}{{0}^{\text{-2}}}\text{N}{{\text{m}}^{\text{-1}}}\]
Correct Answer: D
Solution :
A soap film has two free surfaces, so total length of the film to be supported, \[l=2\times 30cm=0.60m.\] Let T = surface tension of the film. If f = total force on the slider due to surface tension, then \[f=T\times 2l=T\times 0.6N\] \[W=1.5\times {{10}^{-2}}N\] In equilibrium position, the force f on the slider due to surface tension must be balanced by the weight (w) supported by the slider i.e., \[\text{f}=\text{w}=\text{mg}\] \[T\times 0.6=1.5\times {{10}^{-2}}\] \[T=\frac{1.5\times {{10}^{-2}}}{0.6}\] \[T=2.5\times {{10}^{-2}}\text{N}{{\text{m}}^{\text{-1}}}\]
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