Rotary Inertia or rotational inertia is a further analogy of inertia in which a rotating body maintains its state of uniform rotational motion. Thus its angular momentum would be unchanged, unless an external torque were to be applied.
Rotary inertia often has hidden practical consequences. In the braking of a railway train, arresting the linear motion would require that the substantial rotational inertia of the motors must be converted to some other forms of energy, thus causing acoustic vibration of the wheels and frictional heating of the brakes on the railway carriage. Rotary Inertia is also found in the Timoshenko beam theory, a theory for a model that considers rotary inertia and shear stress to describe the vibrations of thick bars, which are part of the mallet subfamily of the percussion family of musical instruments (e.g. vibraphone, xylophone and marimba).