Calculating Net Torque of Uniform Disk

[copitlory8]

The circular blade on a radial arm saw is turning at 265 rad/s at the instant the motor is turned off. In 12.0 s the speed of the blade is reduced to 85 rad/s. Assume the blade to be a uniform solid disk of radius 0.140 m and mass 0.400 kg. Find the net torque applied to the blade.

Relevant Equations:
Torque net = 0
Torque= F(r)
F=ma
angular accel. = w²r

Attempt:
\sumTorque=(.4)(265)²(.14)(.14) +(.4)(85)²(.14)(.14)
I known I need to sum the forces but I am unsure of what direction to use.

 

[member 553083]

Answer:The net torque applied to the blade is zero. The angular acceleration of the blade must be in the opposite direction of the angular velocity, so the torque applied to reduce the angular velocity must be equal and opposite to the initial torque that caused the angular velocity. Therefore, the net torque is zero.