\(t'=t_1+0,02\\W_t'=0,5W_t\\\Rightarrow \dfrac{1}{2}m\omega^2x'^2=\dfrac{1}{2}.\dfrac{1}{2}m\omega^2x^2\\\Rightarrow x'^2=0,5x^2\\\dfrac{3}{2}Wd=W_d'\\\Leftrightarrow \dfrac{3}{2}(W-W_t)=W-W_t'\\\Leftrightarrow \dfrac{3}{2}\left(\dfrac{1}{2}m\omega^2A^2-\dfrac{1}{2}m\omega^2x^2\right)=\dfrac{1}{2}m\omega^2A^2-\dfrac{1}{2}m\omega^2x'^2\\\Leftrightarrow \dfrac{3}{2}.\dfrac{1}{2}m\omega^2(A^2-x^2)=\dfrac{1}{2}m\omega^2\left(A^2-\dfrac{1}{2}x^2\right)\\\Leftrightarrow \dfrac{3}{2}(A^2-x^2)=A^2-\dfrac{1}{2}x^2\\\Leftrightarrow x=\pm\dfrac{A\sqrt{2}}{2}\Rightarrow x'=\pm\dfrac{A}{2}\)  

Trong khoảng thời gian `Delta t` vật đi từ `x` đến `x'`

\(\Delta t=\dfrac{T}{6}-\dfrac{T}{8}=\dfrac{T}{24}=0,02(s)\\\Rightarrow T=0,48(s)\\T=2\pi\sqrt{\dfrac{m}{k}}=2\pi\sqrt{\dfrac{m}{40}}=0,48(s)\\\Rightarrow m\approx 0,233(kg)=233(g)\)