For very small mass differences between m 1 and m 2, the rotational inertia I of the pulley of radius r cannot be neglected. The angular acceleration of the pulley is given by the no-slip condition: α α = a r , where α α is the angular acceleration. The net torque is then: τ τ n e t = ( T 1 − − T 2 ) r − − τ τ f r i c t i o n = I α α