# Instead of a dynamo-meter, why can't we use $\tau=I\alpha$ to determine the power of a rotating shaft?

To measuring the power of a rotating shaft, dynamo-meters are used. Instead, why can't we find torque using Torque=Moment of Inertia times Angular Acceleration, $\tau=I\alpha$? And given that we know the rpm, $N$,

$$P=\frac{2\pi N\tau}{60}$$

Isn't this a more linear approach for power measurement?