(b) $v_{1} = \frac{dy_{1}}{dt}$
$= 0.1 \times 100\pi\cos{(100\pi t + \pi/3)}$
$v_{2} = \frac{dy_{2}}{dt} = 0.1 \times 100\pi\sin{100\ \pi t}$
$= 0.1 \times 100\pi\cos{(100\pi t + \pi/2)}$
Phase difference between two velocities
$(100\pi t + \pi/2) - (100\pi t + \pi/3)$
$= \pi/2 - \pi/3 - \pi/6$