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Mechanical Properties of Fluids — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysicsMechanical Properties of Fluids5 Marks
Question
Figure (a) shows a thin liquid film supporting a small weight = $4.5 \times 10^{–2}N$. What is the weight supported by a film of the same liquid at the same temperature in Fig. (b) and (c)? Explain your answer physically.
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Answer

Take case (a): The length of the liquid film supported by the weight, l = 40cm = 0.4cm The weight supported by the film, $W = 4.5 \times 10^{–2}N$ A liquid film has two free surfaces. Surface tension $=\frac{\text{W}}{2\text{l}}=4.5\times\frac{10^{-2}}{2\times0.4}=5.625\times10^{-2}\text{Nm}^{-1}$ In all the three figures, the liquid is the same. Temperature is also the same for each case. Hence, the surface tension in figure (b) and figure (c) is the same as in figure (a), i.e., $5.625 \times 10^{–2}N m^{–1}$. Since the length of the film in all the cases is 40cm, the weight supported in each case is $4.5 \times 10^{–2}N$.

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