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 $B$

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