Câu hỏi 25

Theo bài ra ta có:

\(\eqalign{
& \left\{ \matrix{
{T_1} = 2\pi \sqrt {{l \over {g + a}}} = 2,52 \hfill \cr
{T_2} = 2\pi \sqrt {{l \over {g - a}}} = 3,15 \hfill \cr} \right. \Rightarrow \left\{ \matrix{
{l \over {g + a}} = {{{{2,52}^2}} \over {4{\pi ^2}}} \hfill \cr
{l \over {g - a}} = {{{{3,15}^2}} \over {4{\pi ^2}}} \hfill \cr} \right. \Rightarrow \left\{ \matrix{
{g \over l} + {a \over l} = {{4{\pi ^2}} \over {{{2,52}^2}}} \hfill \cr
{g \over l} - {a \over l} = {{4{\pi ^2}} \over {{{3,15}^2}}} \hfill \cr} \right. \cr
& \Rightarrow {g \over l} = 2{\pi ^2}\left( {{1 \over {{{2,52}^2}}} + {1 \over {{{3,15}^2}}}} \right) \Rightarrow {l \over g} = {1 \over {2{\pi ^2}\left( {{1 \over {{{2,52}^2}}} + {1 \over {{{3,15}^2}}}} \right)}} \cr} \)

 Khi thang máy đứng yên: \(T = 2\pi \sqrt {{\ell  \over g}}  = 2,78s\)