Mechanical Properties of Fluids

Question : 17

Total: 31

A U-shaped wire is dipped in a soap solution, and removed. The thin soap film formed between the wire and the light slider supports a weight of 1.5×10–2 N (which includes the small weight of the slider). The length of the slider is 30 cm. What is the surface tension of the film?

Solution:

We know that a soap film has two free surfaces, so total length of the film to be supported, l=2×30cm
or l=60cm=0.60m
Let T = surface tension of the film
If F = total force on the slider due to surface tension, then
F=T×2l=T×0.6N
W=1.5×10–2N
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. F=W=mg
or T×0.6=1.5×10–2
∴T=

1.5×10−2

0.6

=2.5×10−2Nm−1

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