Acceleration of bouncing balls in external fields
  We introduce two models, the Fermi-Ulam model in an external field and a one dimensional system of bouncing balls in an external field above a periodically oscillating plate. For both models we investigate the possibility of unbounded motion. In a special case the two models are equivalent. 
Tyll Kr\"uger, L. D. Pustyl'nikov, Serge Troubetzkoy