Answer
$\frac{14\pi}{15}\ mm/sec$
Work Step by Step
The second hand of a clock moves $2\pi$ radians per $60$ seconds, thus $\omega=\frac{2\pi}{60}=\frac{\pi}{30}\ rad/sec$ and the linear speed is $v=r\omega=28\times\frac{\pi}{30}=\frac{14\pi}{15}\ mm/sec$
