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