There is a problem that asks for forces that apply torque around the axis of rotation on a spool of string that falls and unwinds under gravity. The spool falls straight down with a vertical string. My query is identifying the axis of rotation, as I understand, it should be passing through the center like in the figure since force due to gravity also passes through the center, and so tension along the string must be the only force that applies torque around the axis of rotation.
But the answer says that
"The spool is rotating around the point where it loses contact with the string. Since the tension force is applied at this point, it creates no torque. The gravitational force is applied at the center of the spool, which is a distance $r$ from the rotation axis, so it applies a torque."
I don't understand how they have chosen the axis of rotation. Can someone explain?