Câu 12:
\(\begin{array}{l}
\left\{ \begin{array}{l}
{v_{\max }} = 80\pi \\
{a_{\max }} = \dfrac{{3,2}}{{0,1}} = 32m/{s^2} = 3200cm/{s^2}
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
\omega A = 80\pi \\
{\omega ^2}A = 3200
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
\omega = 4\pi \\
A = 20cm
\end{array} \right.
\end{array}\)
Tại t = 1,25s:
\(\begin{array}{l}
x = 20\cos \left( {4\pi .1,25 + \varphi } \right) = 10\\
\Rightarrow \cos \varphi = - \dfrac{1}{2}\\
\Rightarrow \varphi = - \dfrac{{2\pi }}{3}\\
\Rightarrow x = 20\cos \left( {4\pi t - \dfrac{{2\pi }}{3}} \right)
\end{array}\)
Câu 13:
\(\begin{array}{l}
\left\{ \begin{array}{l}
{v_{\max }} = 100\\
{a_{\max }} = 1000cm/{s^2}
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
\omega A = 100\\
{\omega ^2}A = 1000
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
\omega = 10\\
A = 10cm
\end{array} \right.
\end{array}\)
Tại \(t = 1,55\pi \left( s \right)\):
\(\begin{array}{l}
t = 1,55\pi \left( s \right)\\
x = 10\cos \left( {10.1,55\pi + \varphi } \right) = 0\\
\Rightarrow \varphi = \pi \\
\Rightarrow x = 10\cos \left( {4\pi t + \pi } \right)
\end{array}\)
