Experimental observation of negative rotational inertia

We report an easy-to-make, resonance-based mechanism to realize negative rotational inertia. The device consists of three parts: a heavy inner core, a lightweight outer shell, and rubber connections between the core and shell. We theoretically predict and experimentally observe the negative rotational inertia in the range of 100-230 Hz. The experimental values are obtained via measurements of vibrational response. We further clarify the relation between the bandwidth of negative inertia and the bandgap in a chain consisting of an array of negative-inertia units. The findings reveal a unique property different from conventional systems in classical physics and offer an opportunity for metamaterial designs.