  We examine the question of whether violation of 4D physics is an inevitable
consequence of existence of an extra non-compactified dimension. Recent
investigations in membrane and Kaluza-Klein theory indicate that when the
metric of the spacetime is allowed to depend on the extra coordinate, i.e., the
cilindricity condition is dropped, the equation describing the trajectory of a
particle in one lower dimension has an extra force with some abnormal
properties. Among them, a force term parallel to the four-velocity of the
particle and, what is perhaps more surprising,
  $u_{\mu}f^{\mu} \neq u^{\mu}f_{\mu}$. These properties violate basic concepts
in 4D physics. In this paper we argue that these abnormal properties are {\em
not} consequence of the extra dimension, but result from the formalism used. We
propose a new definition for the force, from the extra dimension, which is free
of any contradictions and consistent with usual 4D physics. We show, using
warp-products metrics, that this new definition is also more consistent with
our physical intuition. The effects of this force could be detected observing
objects moving with high speed, near black holes and/or in cosmological
situations.
