Câu 46:
$T=2\pi\sqrt{\dfrac{m}{k}}$
$k=\dfrac{mg}{l}\to T=2\pi\dfrac{l}{g}$
Khi $l'=\dfrac{l}{2}$
$\to T'=2\pi\dfrac{l}{2g}=\dfrac{1}{\sqrt2}T$
Chọn $D$
Câu 47:
$f=\dfrac{1}{2\pi}\sqrt{\dfrac{k}{m}}$
Khi $k'=2k; m'=\dfrac{m}{8}$
$\to f'=\dfrac{1}{2\pi}\sqrt{ \dfrac{2k.8}{m}}=4f$
Chọn $D$
Câu 48:
$\dfrac{T'}{T}=\sqrt{ \dfrac{k}{k'}}=\dfrac{1}{4}$
$\to k'=16k$
$\to l'=\dfrac{l}{16}$
Chọn $D$
