1.

A U-shaped wire is dipped in a soap solution, and removed. A thin soap film formed between the wire and a light slider supports a weight of `1.5xx10^(-2)N` (which includes the small weigh of the slider). The length of the slider is 30cm. What is the surface tension of the film?

Answer» We know that soap film has two free surfaces , so total length of the film to be supported,
`2l = 2xx30=60cm =0.6m`
Total force on the slider due to surface tension will be , `F = S xx 2l = S xx 0.6N`
In equilibrium position, the force F on slider due to surface tension must balance the weight mg
`(=1.5xx10^(2)N)` , i.e, `F=1.5xx10^(-2) = Sxx0.6 or S=(1.5xx10^(-2))/(0.5) = 2.5 xx 10^(-2)Nm^(-1)`.


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