Answer
$1.1 \times 10^{-2}N$
Work Step by Step
As explained in the text, in figure 10–34 there is both a top and a bottom surface.
$$\gamma = \frac{F}{2 \mathcal{l}}$$
$$F=2 \gamma \mathcal{l}=2(0.025 N/m)(0.215m) =1.1 \times 10^{-2}N$$
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