Adisk of radius r = 11 cm is pulled along a frictionless surface with a force of f = 16 n by a string wrapped around the edge. at the instant when d = 30 cm of string has unwound off the disk, what is the torque exerted about the center of the disk?

1.76 Nm

Explanation:

If the force pulls horizontally, this means that the force is tangent to the disk at any point of the string unwinding process, so the distance d is irrelevant.

In this case, the torque is directly given by the product of the force times the distance perpendicular to the center of the disk, which is just the radius, as follows:

τ = F * r = 16 N. (0.11) m = 1.76 Nm