  In the present paper we consider a 3-dimensional differentiable manifold $M$
equipped with a Riemannian metric $g$ and an endomorphism $Q$, whose third
power is the identity and $Q$ acts as an isometry on $g$. Both structures $g$
and $Q$ determine an associated metric $f$ on $(M, g, Q)$. The metric $f$ is
necessary indefinite and it defines isotropic vectors in the tangent space
$T_{p}M$ at an arbitrary point $p$ on $M$.
  The physical forces are represented by vector fields. We investigate physical
forces whose vectors are in $T_{p}M$ on $(M, g, Q)$. Moreover, these vectors
are isotropic and they act along isotropic curves. We study the physical work
done by such forces.
