\(v\left(t\right)=s'\left(t\right)=3cos\left(t+\dfrac{\pi}{3}\right)\)
\(a\left(t\right)=v'\left(t\right)=-3sin\left(t+\dfrac{\pi}{3}\right)\)
\(\Rightarrow a\left(\dfrac{\pi}{2}\right)=-3sin\left(\dfrac{\pi}{2}+\dfrac{\pi}{3}\right)=-\dfrac{3}{2}\) \(\left(m/s^2\right)\)