A string is wrapped around the rim of a flywheel 0.400 m in radius, and a steady pull of 50.0 N is exerted on the cord. The wheel is mounted in frictionless bearings on a horizontal shaft through it's center. The moment of inertia of the wheel is 4.00 kg.m^2 .Calculate the angular acceleration of the wheel.

2 answers

Read up examples in
http://spiff.rit.edu/classes/phys211/lectures/rotke/rotke_all.html
one of which is similar to this one.

After this, you can attempt the previous problem you posted, together with your previous knowledge of kinematics and energy principles.
The torque T = 50N*0.4m
= 20N.m
The moment of inertia is I = 4.00kg.m^2.
The angular acceleration in rad/s^2 alpha = T/I
=20N.m/(4.00kg/m^2)
=5 rad/s^2