Ghép $i$ lò xo song song: $k_{tđ} =k_1+k_2+k_3+...+k_i$
Ghép $n$ lò xo nối tiếp: $\dfrac{1}{k_{tđ}}=\dfrac{1}{k_1}+\dfrac{1}{k_2}+...+\dfrac{1}{k_n}$
Câu 57:
$\dfrac{1}{k_{tđ}}=\dfrac{5}{k}$
$\to k_{tđ}=\dfrac{k}{5}$
$\to f'=\dfrac{1}{2\pi}.\sqrt{\dfrac{k_{tđ}}{m}}=\dfrac{1}{\sqrt5}.\sqrt{\dfrac{k}{m}}=\dfrac{f}{\sqrt5}$
Chọn $B$
Câu 58:
$T=2(s)$
$k_{tđ}=k+k=2k$
$\to T'=2\pi\sqrt{ \dfrac{m}{k_{tđ}}}=2\pi.\sqrt{ \dfrac{m}{2k}}=\dfrac{T}{\sqrt2}=\sqrt2(s)$
Chọn $D$
Câu 59:
$\dfrac{1}{k_{tđ}}=\dfrac{1}{k_1}+\dfrac{1}{k_2}$
$\to k_{tđ}=\dfrac{k_1k_2}{k_1+k_2}$
$T_1=2\pi\sqrt{\dfrac{m}{k_1}}$
$\to 0,6^2=2^2.10.\dfrac{m}{k_1}$
$\to k_1=\dfrac{1000m}{9}$
$T_2=2\pi.\sqrt{\dfrac{m}{k_2}}$
$\to 0,8^2=2^2.10.\dfrac{m}{k_2}$
$\to k_2=62,5m$
$\to k_{tđ}=40m$
$\to T=2\pi\sqrt{\dfrac{m}{k_{tđ}}}=1(s)$
Chọn $B$
