These are the words from 3rd paragraph of Topic "The Conical Pendulum" in book Halliday Resnink Krane 5th-e "The radial component is negative if we define the radial direction to be positive outward from the central axis" What is central axis here. Is it center of bob or center of circle? if center of bob then then it means that we take direction along normal of the tangent to the circle but if center of circle then it is confusing. Now In the next topic named "Rotor" in second paragraph the writer says "we define the positive radial direction outward from the axis of rotation". here it has made reference "axis of rotation" mean center of rotor. My question is "How we will take the direction of radial component or in other words what is the origin of cartesian coordinates (reference point)? As in conical pendulum the reference is center of bob(in my view) but in rotor the reference is center of axis of rotation
I don't see Bob in the pendulum problem but I assume he is the sphere attached to the string.
The central axis is clearly that of rotation (the vertical axis). The radial coordinate of Bob is simply defined as the shortest distance between Bob and that axis.
If Bob goes closer to the axis, its change in radial coordinate will be negative.