Đáp án: `(B) \ 0,5s`
Giải:
`Δt=2 \ ph=120 \ s`
`T=\frac{Δt}{n}=\frac{120}{120}=1 \ (s)`
Ta có:
`T=2π\sqrt{\frac{l}{g}}` (1)
$T'=2π\sqrt{\dfrac{l'}{g}}=2π\sqrt{\dfrac{\dfrac{l}{4}}{g}}$ (2)
Lấy (1) chia (2) vế theo vế
→ $\dfrac{T}{T'}=\sqrt{\dfrac{l}{\dfrac{l}{4}}}=\sqrt{4}=2$
→ $\dfrac{1}{T'}=2$
→ $T'=0,5 \ (s)$