$= a \times \frac{{2\pi }}{T} $
$= (50 \times {10^{ - 3}}) \times \frac{{2\pi }}{2}$
$= 0.15\,m/s$

$(A)$ The force is zero $t=\frac{3 T}{4}$
$(B)$ The acceleration is maximum at $t=T$
$(C)$ The speed is maximum at $t =\frac{ T }{4}$
$(D)$ The $P.E.$ is equal to $K.E.$ of the oscillation at $t=\frac{T}{2}$