A fixed disc free to rotate about its center (a flywheel) has a string wrapped around it, with a block attached to it. As the block falls, The tension in the string makes the disc rotate. Now, when the disc rotates by an angle $\theta$ (assuming the string doesn't slip), a length $r\theta$ of string unwraps. The length of the piece of string which is attached to the block is changing. Why is work done by tension (on block + disc system) zero?
I don't get it.
So if I'm using the energy theorem in this system that states:
Variation of energy = Work done by string tension (considering that the tension here is the only non conservative force) = 0?! Because work done by tension is zero?
[Here is the system]