Inverted pendulum.

Inverted (Kapista) Pendulum. A usual pendulum has two equilibrium points, one stable, and the other unstable (pointing up). However, if a fast periodic vertical force is applied to the oscillating weight, when the force is sufficiently strong, the normally unstable equilibrium point also becomes stable, and oscillation about this point can be observed. This demonstration shows this effect in action(and is quite impressive for those who have not seen it before). The stabilization effect illustrated by this demonstration is well known not only in mechanics, but also in the heart of quadrupole mass-spectrometers and the so-called Paul traps for charged particles. It is also widely used in particle accelerators, for focusing and stabilizing charged particles.