Question of puzzler4:
Are there any more impact laws besides Newton's Impact law: v^+_rel=-e*v^-_rel ?
Answer:
For instance Poisson’s impact law, which requires a decomposition of the impact process
into a compression phase (-) and a decompression phase (+) in order to define the
restitution coefficient e by the ratio of the corresponding impulsive forces,
p^+=-e*p^-

ref:
@ARTICLE{Glocker2001,
  author = {Glocker, C.},
  title = {On frictionless impact models in rigid-body systems},
  journal = {Philosophical Transactions of the Royal Society London A},
  year = {2001},
  volume = {359},
  pages = {2385--2404},
  abstract = {The paper reviews the frictionless collision problem in rigid-body
	dynamics. After having established the contact kinematical equations
	and the impact equations, Newton’s and Poisson’s impact laws are
	stated in inequality form for one collision point. This situation
	is extended by superposition to multi-contact configurations. The
	concept of superposition together with the equations for a single
	contact defines the mechanical impact theory. It is valid only within
	a certain restricted framework specified by several physical assumptions,
	which are discussed to some extent.},
  doi = {10.1098/rsta.2001.0857},
  file = {Glocker2001.pdf:Glocker2001.pdf:PDF},
  owner = {schwab},
  timestamp = {2009.03.12}
}