@book{BackhausSchoen2001,
  author    = {Backhaus, Udo and Sch{\"o}n, Lutz-Helmut},
  title     = {Physik plus : Kl. 8, [Hauptbd.], Gymnasium Nordrhein-Westfalen},
  editor    = {Mikelskis, Helmut and Wilke, Hans-Joachim},
  publisher = {Volk und Wissen Verl.},
  address   = {Berlin},
  isbn      = {3-06-020840-9},
  pages     = {136 S.},
  year      = {2001},
  language  = {de}
}
@book{Backhaus2002,
  author    = {Backhaus, Udo},
  title     = {Physik plus : Kl. 9/10, [Hauptbd.], Gymnasium Nordrhein-Westfalen},
  editor    = {Mikelskis, Helmut and Wilke, Hans-Joachim and Sch{\"o}n, Lutz-Helmut},
  publisher = {Volk und Wissen},
  address   = {Berlin},
  isbn      = {3-06-020980-4},
  pages     = {244 S.},
  year      = {2002},
  language  = {de}
}