\(x=10 \cos \left[2 \pi t+\frac{\pi}{2}\right]\)
At \(t=\frac{1}{6} s\)
\(x=10 \cos \left[\frac{\pi}{2}+\frac{\pi}{3}\right]\)
\(x=-10 \sin \frac{\pi}{3}\)
\(x=-5 \sqrt{3}\)
\(v=\omega \sqrt{A^2-x^2}\)
\(v=2 \pi \sqrt{100-75}\)
\(v=10 \pi\)
\(v=31.4 \,cm / s\)