We consider a simplified Newton's cradle, which would be unobservable if the non-smooth impacts between the spheres were ignored. Nevertheless, under the assumption that there is an infinite sequence of nonsmooth impacts, it is possible to design an observer that is able to asymptotically estimate all the nonmeasurable state variables, including those that would be unobservable in absence of impacts. © 2003 Elsevier Science Ltd. All rights reserved.
Menini, L., & Tornambè, A. (2003). State estimation for the Newton’s cradle: A mechanism that is unobservable in absence of impacts. Applied Mathematics Letters, 16(4), 469–474. https://doi.org/10.1016/S0893-9659(03)00022-3