text: physics grindstone question? a disk-shaped grindstone of mass 1.2kg and radius 6cm is spinning at 750 rev/min.  after the power is shut off, a woman continues to sharpen her ax by holding it against the grindstone for 10s until the grindstone stops rotating. (assume constant angular acceleration and a lack of other frictional torques.  include the sign in your answers)  (a) what is the angular acceleration of the grindstone?  (b) what is the torque exerted by the ax on the grindstone? 750rpm is 25*pi radians per second and the disc slows down from this to 0 in 10 seconds - you can work out what the acceleration was and whether it is -ve or +ve  somewhere you probably have a table showing moments of inertia of common symmetrical shapes. if not you can find one here - http://eta.physics.uoguelph.ca/tutorials/torque/q.torque.inertia.html  find the formula for a disc or cylinder rotating about the axis. this gives you i. (convert cms to metres when you work out i to be sure you will get torque in n.m at the end).  now you find the torque using w_final = w_initial + torque*i  good luck