ω = v / r
Explanation:
Since the cylinder is wrapped about the string, it rolls without slipping with respect to the string. The constraint relationship to assure this kind of motion can be obtained by considering the relative velocity of the point on the cylinder that is in contact with the string with respect to the stringit should be zero.
The velocity of the center of the cylinder,
v = dy (t) / dt, r and the rotation rate of the cylinder, ω
We have to first find the velocity of the contact point relative to the center of the cylinder. Since this ignores the translational motion of the entire cylinder with respect to the ground, we then need to add back in the cylinder's center of mass velocity to find the velocity of the contact point with respect to the ground.
So,
v (contact) = ωr - v
where,
ω is the angular velocity
v is the linear velocity
r is the radius
ω = v / r