Here is another problem I'm having trouble understanding: A narrow but solid spool of thread has radius R and mass M. If you pull up on the thread so that the center of mass of the spool remains suspended in the air at the same place, (a) what force must you exert on the thread? (b) How much work have you done by the time the spool turns with angular velocity w? How is the center of mass not moving when I'm pulling on a thread and effectively removing thread (a therefore mass) from the spool. And how can it remain suspended in the air? I just can't picture this. How is it possible to do work when the spool hasn't rotated yet?