A uniform disc of mass M and radius R is mounted on an axle supported in frictionless bearings. A light cord is wrapped around the rim of the disc and steady downward pull T is exerted on the cord. The angular acceleration of the disc is : 

\(\frac{2T}{MR}\)

Torque exerted on the disc : \(\tau\)

\(\tau = TR\)

Now, \(\tau = I \alpha\)

\(\alpha = \frac{\tau}{I}\)

\(\alpha = \frac{TR}{\frac{1}{2} MR^2}\)

\(\alpha = \frac{2T}{MR}\)