Câu 7:
Ta có:
\(\begin{array}{l}
{l_1} - {l_2} = 1 \Rightarrow {l_0}\left( {1 + 100{\alpha _1}} \right) - {l_0}\left( {1 + 100{\alpha _2}} \right) = 1\\
\Rightarrow {l_0} = 442,5mm
\end{array}\)
Câu 8:
Để thanh cân bằng:
\(P = F = 2\sigma l = 2.0,04.\dfrac{{50}}{{1000}} = {4.10^{ - 3}}N\)