A rectangular element with thickness \(1.0 \mathrm{~m}\) is under gravity. Express strain \(\varepsilon_{y y}\) in terms of vertical nodal displacement \(-v\) at the top. \(\left(v_{1}=v_{2}=0, v_{3}=v_{4}=-vight)\). Explain if the calculated strain \(\varepsilon_{y y}\) is exact or not. (If you need, use \(g\) : gravitational acceleration, \(ho\) : density, \(E\) : elastic modulus).

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