Question

# A $$10\ cm$$ long wire is placed horizontally on the surface of water and is gently pulled up with a force of $$\displaystyle 2\times { 10 }^{ -2 }N$$. To keep the wire in equilibrium, the surface tension of water in $$\displaystyle { Nm }^{ -1 }$$ is.

A
0.1
B
0.2
C
0.001
D
0.002

Solution

## The correct option is A $$0.1$$As the liquid film has 2 surfaces hence,$$\displaystyle F=S \times 2l$$$$\displaystyle \Rightarrow S=\frac { F }{ 2l } =\frac { 2\times { 10 }^{ -2 }N }{ 2\times \left( 10\times { 10 }^{ -2 }m \right) }$$$$\displaystyle S={ 10 }^{ -1 }{ N }/{ m }=0.1{ N }/{ m }$$Physics

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