The phase space idea can even be applied to differential equations
with order higher than two. For example, a constraint may involve the
time derivative of acceleration , which is often called *jerk*. If the differential
equations involve jerk variables, then a -dimensional phase space
can be defined to obtain first-order constraints. In this case, each
,
, and
in a constraint such as
is defined as a phase variable.
Similarly, th-order differential constraints can be reduced to
first-order constraints by introducing a -dimensional phase space.

A -dimensional phase space is defined in which

(13.36) |

The state (or phase) transition equation is for each such that , and . Together, these individual equations are equivalent to .

The initial state specifies the initial position and all time
derivatives up to order . Using these and the action , the
state trajectory can be obtained by a chain of integrations.

You might be wondering whether derivatives can be eliminated completely by introducing a phase space that has high enough dimension. This does actually work. For example, if there are second-order constraints, then a -dimensional phase space can be introduced in which . This enables constraints such as to appear as . The trouble with using such formulations is that the state must follow the constraint surface in a way that is similar to traversing the solution set of a closed kinematic chain, as considered in Section 4.4. This is why tangent spaces arose in that context. In either case, the set of allowable velocities becomes constrained at every point in the space.

Problems defined using phase spaces typically have an interesting
property known as *drift*. This means that for some ,
there does *not* exist any such that
. For
the examples in Section 13.1.2, such an action always
existed. These were examples of *driftless
systems*. This was possible because the
constraints did not involve dynamics. In a dynamical system, it is
impossible to instantaneously stop due to momentum, which is a form of
drift. For example, a car will ``drift'' into a brick wall if it is
meters way and traveling 100 km/hr in the direction of the wall.
There exists no action (e.g., stepping firmly on the brakes) that
could instantaneously stop the car. In general, there is no way to
instantaneously stop in the phase space.

Steven M LaValle 2012-04-20