Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass
We analyze the dynamical equations obeyed by a classical system with position-dependent mass. It is shown that there is a non-conservative force quadratic in the velocity associated to the variable mass. We construct the Lagrangian and the Hamiltonian for this system and find the modifications requi...
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| Опубліковано в: : | Symmetry, Integrability and Geometry: Methods and Applications |
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Інститут математики НАН України
2013
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| Цитувати: | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass / Sara Cruz y Cruz, Oscar Rosas-Ortiz // Symmetry, Integrability and Geometry: Methods and Applications. — 2013. — Т. 9. — Бібліогр.: 46 назв. — англ. |
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| author | Sara Cruz y Cruz Rosas-Ortiz, Oscar |
| author_facet | Sara Cruz y Cruz Rosas-Ortiz, Oscar |
| citation_txt | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass / Sara Cruz y Cruz, Oscar Rosas-Ortiz // Symmetry, Integrability and Geometry: Methods and Applications. — 2013. — Т. 9. — Бібліогр.: 46 назв. — англ. |
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| container_title | Symmetry, Integrability and Geometry: Methods and Applications |
| description | We analyze the dynamical equations obeyed by a classical system with position-dependent mass. It is shown that there is a non-conservative force quadratic in the velocity associated to the variable mass. We construct the Lagrangian and the Hamiltonian for this system and find the modifications required in the Euler-Lagrange and Hamilton's equations to reproduce the appropriate Newton's dynamical law. Since the Hamiltonian is not time invariant, we get a constant of motion suited to write the dynamical equations in the form of the Hamilton's ones. The time-dependent first integrals of motion are then obtained from the factorization of such a constant. A canonical transformation is found to map the variable mass equations to those of a constant mass. As particular cases, we recover some recent results for which the dependence of the mass on the position was already unnoticed, and find new solvable potentials of the Pöschl-Teller form which seem to be new. The latter are associated to either the su(1,1) or the su(2) Lie algebras depending on the sign of the Hamiltonian.
|
| first_indexed | 2025-12-02T04:27:21Z |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 9 (2013), 004, 21 pages
Dynamical Equations, Invariants and Spectrum
Generating Algebras of Mechanical Systems
with Position-Dependent Mass?
Sara CRUZ Y CRUZ † and Oscar ROSAS-ORTIZ ‡
† SEPI-UPIITA, Instituto Politécnico Nacional, Av. IPN No. 2580,
Col. La Laguna Ticomán, C.P. 07340 México D.F. Mexico
E-mail: sgcruzc@ipn.mx
‡ Physics Department, Cinvestav, A.P. 14740, México D.F. 07000, Mexico
E-mail: orosas@f is.cinvestav.mx
Received July 31, 2012, in final form January 12, 2013; Published online January 17, 2013
http://dx.doi.org/10.3842/SIGMA.2013.004
Abstract. We analyze the dynamical equations obeyed by a classical system with position-
dependent mass. It is shown that there is a non-conservative force quadratic in the velocity
associated to the variable mass. We construct the Lagrangian and the Hamiltonian for this
system and find the modifications required in the Euler–Lagrange and Hamilton’s equations
to reproduce the appropriate Newton’s dynamical law. Since the Hamiltonian is not time
invariant, we get a constant of motion suited to write the dynamical equations in the form
of the Hamilton’s ones. The time-dependent first integrals of motion are then obtained
from the factorization of such a constant. A canonical transformation is found to map the
variable mass equations to those of a constant mass. As particular cases, we recover some
recent results for which the dependence of the mass on the position was already unnoticed,
and find new solvable potentials of the Pöschl–Teller form which seem to be new. The latter
are associated to either the su(1, 1) or the su(2) Lie algebras depending on the sign of the
Hamiltonian.
Key words: Pöschl–Teller potentials; dissipative dynamical systems; Poisson algebras; clas-
sical generating algebras; factorization method; position-dependent mass
2010 Mathematics Subject Classification: 35Q99; 37J99; 70H03; 70H05
1 Introduction
In recent papers [13, 14, 15, 29] the factorization of the classical Hamiltonian in terms of two
functions that, together with the Hamiltonian itself, lead to a Poisson algebra was discussed.
This program included mechanical systems with mass explicitly dependent on position [14, 15]
and was extended to the quantum case [14, 15, 16, 17] (other already reported approaches can
be found in [1, 4, 37]). Position-dependent mass functions m(x) give rise to ‘forces quadratic
in the velocity’ which, in turn, lead to nonlinear differential equations of motion in the New-
tonian approach. The most celebrated example of this kind of equations, due to Mathews and
Lakshmanan [33] (see also [8, 10]), corresponds to the nonlinear oscillator(
1 + λx2
)
ẍ− λxẋ2 + α2x = 0, (1.1)
and is derivable from the Lagrangian
L =
1
2
(
1
1 + λx2
)(
ẋ2 − α2x2
)
.
?This paper is a contribution to the Special Issue “Superintegrability, Exact Solvability, and Special Functions”.
The full collection is available at http://www.emis.de/journals/SIGMA/SESSF2012.html
mailto:sgcruzc@ipn.mx
mailto:orosas@f\/is.cinvestav.mx
http://dx.doi.org/10.3842/SIGMA.2013.004
http://www.emis.de/journals/SIGMA/SESSF2012.html
2 S. Cruz y Cruz and O. Rosas-Ortiz
Masses varying with the position can be also associated to the kinetic energy of dynamical
systems in curved spaces with either constant curvature [9, 27, 41], or non-constant curvatu-
re [5, 38]. Similar relationships arise in geometric optics where the position dependent refractive
index can be interpreted as a variable mass [46]. In semiconductor theory, it has been found
that the coherent superpositions of states connected to different masses are forbidden [44].
Moreover, there is not unicity in the construction of quantum Hamiltonians (see, e.g., [14, 16]
and references quoted therein), and the invariance under the change of inertial frames is not
granted for position-dependent mass systems [45] (however, see the discussion on the matter
in [32]). In general, the variable mass dynamics involves masses which are functions of either
position or time (or functions of time and position). Besides the works mentioned above, the
applications include the motion of rockets [42], the raindrop problem [28], the variable mass
oscillator [21], the inversion potential for NH3 in density theory [2], the two-body problem
associated to the evolution of binary systems [26], the effects of galactic mass loss [39], neutrino
mass oscillations [6], and the problem of a rigid body against a liquid free surface [35] among
other. Yet, the discussion is far from being exhausted. For instance, it has been indicated that
Newton’s second law is valid only for constant mass and that, if the mass variation is due to
accretion or ablation, the corresponding equation must be modified [36]. Other works report
the conservation laws for a variable mass system [18] and the construction of standard and
non-standard Lagrangians for dissipative variable mass systems [34].
The present work deals with the dynamical equations obeyed by classical position-dependent
mass systems. We analyze the problem in the Newton, Lagrange and Hamilton approaches by
deriving in each case the equations of motion. A non-conservative force which is quadratic in the
velocity appears because of the mass-function m(x) and induces modifications in the dynamical
laws with respect to the constant mass case. As a consequence, the position-dependent mass
Hamiltonian H = p2
2m(x) + V(x) is not time-independent. Hence, the force due to the variable
mass can be associated to dissipation. Despite this result, it is possible to construct a constant of
motion H (in energy units) that allows to express the dynamical equations in the conventional
Hamilton’s form for the appropriate phase space. Using this invariant and the factorization
method the problem can be solved in an algebraic form.
The organization of the paper is as follows. In Section 2, we depart from the Newton’s equa-
tion of motion for these systems and show that it is possible to get a Lagrangian in the standard
form L = p2
2m(x) − V(x), at the cost of modifying the Euler–Lagrange equations of motion. The
modification must include the non-inertial force which is quadratic in the velocity. Then, the
variable mass Hamiltonian H is constructed and the related equations of motion are shown to
be a modified version of the conventional Hamilton’s equations while they are consistent with
both, the Newtonian and the Lagrangian formulations already discussed. It is shown that the
time derivative of the Hamiltonian H is cubic in the velocity. In Section 2.2.1 we construct the
constant of motion H and show that the related dynamical equations acquire the form of the
conventional Hamilton’s equations. Although the Lagrangian and Hamiltonian are not unique,
our expressions for L, H, and H are found to be in correspondence with the general expressions
derived by other authors [34]. In Section 2.2.2, some other results already reported for constant
masses in presence of forces quadratic in the velocity are shown to be particular cases of our ap-
proach for specific nontrivial forms of the mass-function m(x). In Section 2.3, the factorization
method discussed in [13, 14, 15, 29] is extended to the case of the variable mass systems we deal
with in this paper. As it will be shown, the underlying Poisson structures lead to a deformed
Poisson algebra only if the potential is of the Pöschl–Teller form. Then, explicit expressions for
the phase trajectories described by these variable masses are found. Section 2.4 is devoted to the
analysis of a point transformation that leads from the variable mass problem to the one of a con-
stant mass. This transformation is shown to be canonical in the sense that it leaves invariant the
Hamilton’s equations of motion here derived. In Section 3 we analyze the diverse Pöschl–Teller
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 3
potentials we have at hand by supplying specific mass-functionsm(x) in our approach. In partic-
ular, we use a doubly singular massmss(x) arising from the analysis of the inversion potential for
NH3 in terms of the density operator [2]. The potentials arising from the singular as well as from
the regular masses which we have used in our previous works are also analyzed. Finally, an expo-
nential mass-function me(x), introduced here to recover the Lagrangian forms reported by other
authors for constant mass systems, gives rise to solvable potentials which, as far as we know,
have been not reported previously in the literature. The paper is closed with some conclusions.
2 Deformed algebras of a particle with position dependent mass
2.1 Newtonian framework
For the sake of motivation consider the one-dimensional motion of a system of mass m(x) > 0,
explicitly dependent on the position x, acted by a force F which in general depends on position x,
velocity ẋ, and time t (the case of n generalized coordinates is straightforward). The Newton’s
equation of motion is
F (x, ẋ; t) =
dp
dt
= m′(x)ẋ2 +m(x)ẍ, (2.1)
where p = m(x)ẋ is the linear momentum. Hereafter ġ and g′ will stand for time and position
derivatives of g respectively. To get (2.1) we have assumed a null velocity vm for the accreted
or emitted mass relative to the system (otherwise, one of the velocities in the quadratic term
m′(x)ẋẋ must be substituted by the displacement vm+ ẋ). Equation (2.1) has the general form
F (x, ẋ; t) = α(x; t)ẍ+ β(x, ẋ; t),
and can be decoupled in the nonlinear system
ẋ = v, v̇ =
1
m(x)
[
F (x, v; t)−m′(x)v2
]
. (2.2)
This defines a path on the plane (x, v) in terms of the parameter t (see, e.g., [3]). The latter
expressions acquire the standard form whenever m′ = 0:
ẋ = v, v̇ =
1
m0
F0(x, v; t). (2.3)
Here the constant m0 > 0 corresponds to the mass of a system subjected to the action of the
force F0. It is possible to make the equivalence between the two systems, (2.2) and (2.3), in
such a way that the variable mass m(x) describes the same path as a given constant mass m0.
For instance, consider the mass
m(x) = −m0[1− kV0(x)] (2.4)
with k a constant and V0(x) a differentiable function. Let E0 = 1/k be the total energy of m0,
then equations (2.2) and (2.3) lead to equivalent paths provided that
F0 = −
2E0
m0v2
F + 2
dV0(x)
dx
.
From this expression, notice in particular that the variable mass (2.4) describes, under its own
inertia (i.e., in absence of the external field F ), exactly the same path as the constant mass m0
under the action of the potential function −2V0(x). This example shows that the mechanical
energy E0 of the properly chosen constant mass system can be used to parameterize the path
4 S. Cruz y Cruz and O. Rosas-Ortiz
described by the variable mass m(x) on the (x, v)-plane (regardless the mechanical energy is not
conserved in the latter system). However, we have to emphasize that this equivalence holds for
the paths only, not for the motion with respect to time [30].
To get some insight on the forces involved in the Newton’s equation (2.1) let us rewrite this
in the standard form
m(x)ẍ = Fnet(x, ẋ; t) ≡ F (x, ẋ; t)−m′(x)ẋ2. (2.5)
Since ẋ2 > 0, the system is accelerated if the rate m′(x) is negative, and decelerated if this is
positive. Thus, the term quadratic in the velocity −m′(x)ẋ2 corresponds to the thrust of the
system, and (2.5) indicates how this alters the velocity ẋ. In this way, the net force Fnet on
a particle suffering a spatial variation of its mass results as a combination of the external force F
and the thrust −m′(x)ẋ2.
Mechanical systems of variable mass are mostly studied in the cases where the mass m is an
explicit function of time m(t). Immediate applications involve rockets and jet engines analyzed
in terms of Newtonian theory [42]. The main difficulty is that not all the involved forces are
derivable from an ordinary potential function or even from a generalized potential. In many
situations of interest, the action of such forces produces the non-conservation of the mechanical
energy (see for instance the analysis of the raindrop problem in [28]). Nonetheless, the dynamics
of time-dependent mass systems can be studied in the Lagrangian approach. Indeed, for these
systems in one degree of freedom and forces independent on velocity, Darboux showed that it
is always possible to construct a Lagrangian [19] (see also [31]). The equation of motion for
some of these systems is connected to that of a pendulum whose length ` varies with time in
the small-angle approximation `θ̈ + 2 ˙̀θ̇ + gθ = 0 (0 < θ << 1). For instance, a time-dependent
mass (undamped) oscillator obeys the equation mẍ + ṁẋ + kx = 0. Here, the counterpart
of θ is the displacement x for the oscillator and the thrust −ṁẋ simulates a damping linear in
the velocity. It is well known that the solutions of this last equation are either decreasing or
increasing oscillatory functions, depending on the sign of ṁ. Interestingly, a simple experimental
setup can be used to show that the standard expression for the oscillator’s frequency ω =
√
k/m
remains valid if the mass is time-dependent [21]; a useful result since the mathematical tools
used to analyze the constant mass oscillators can also be applied if the mass depends on time.
Our approach considers a mass which is an explicit function of position m(x), so that implicit
dependence on time is assumedm(x(t)). According to (2.1), the equation of motion of a position-
dependent mass oscillator is nonlinear m(x)ẍ+m′(x)ẋ2+kx = 0 (compare with equation (1.1)).
In contraposition to the time-dependent mass case, the thrust −m′(x)ẋ2 is here quadratic in
the velocity. We stress that, in general, it is not evident how a set of canonical variables can be
found such that the Newton’s equation (2.1) is fulfilled.
In the sequel we shall introduce a mechanism to determine the phase trajectories (x(t), p(t))
associated to the Newton’s equation of motion (2.1) for interactions F derivable from a properly
chosen potential function. We will focus on the invariants which arise after factorizing the related
Hamiltonian. With this aim, we first analyze the problem in the Lagrangian and Hamiltonian
frameworks to determine whether the standard expression of the Hamiltonian H = p2
2m + V
remains valid if the mass depends on position.
2.2 Canonical framework
Departing from the Newton’s equation of motion (2.1) for a force independent on velocity, and
applying the D’Alembert’s principle we arrive at
d
dt
(
∂T
∂ẋ
)
− ∂T
∂x
= F (x; t) + R̃, (2.6)
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 5
with T and R̃ the kinetic energy and reacting thrust, respectively given by
T :=
1
2
m(x)ẋ2, R̃(x, ẋ; t) := −1
2
m′(x)ẋ2.
When the external force is derivable from a scalar potential function F = −∂V
∂x , and this last
does not depend on either velocity or time, equation (2.6) becomes
d
dt
(
∂L
∂ẋ
)
− ∂L
∂x
= R̃, L = T − V. (2.7)
This last is the Lagrangian form of the Newton’s equation of motion (2.1). To verify that the
Lagrange equation (2.7) encodes the dynamics of the variable mass m(x) we are dealing with,
consider that the acceleration a = ẍ arising from L satisfies
Ma = Q, (2.8)
with M = ∂2L
∂2ẋ
the Hessian (matrix) of L with respect to the velocity, and the “Lagrangian
force” Q defined as
Q =
∂L
∂x
− ẋ ∂2L
∂x∂ẋ
+ R̃. (2.9)
The force Q includes the applied (physical) force −∂V
∂x , and the fictitious (non-inertial) force
∂T
∂x − ẋ ∂2T
∂x∂ẋ = −R̃. This last shows that the reacting thrust R̃ is a non-inertial force. The
substitution of L = 1
2m(x)ẋ2 − V(x) in (2.8) and (2.9) reproduces the dynamical law (2.1).
Moreover M = m(x) > 0, so that the Lagrangian L is regular and equation (2.8) can be solved
for the acceleration a = Q/m(x). The solutions x(t) of the second order differential equation
ẍ = a(x, ẋ; t) can be obtained from the nonlinear system (2.2).
Once we have constructed the Lagrangian L for the position-dependent mass m(x), we use
the momentum p = ∂L
∂ẋ = m(x)ẋ to obtain the Hamiltonian H from the Legendre transformation
H(x, p; t) = pẋ− L(x, ẋ; t) = p2
2m(x)
+ V(x). (2.10)
The Hamiltonian’s time rate of change
d
dt
H = R̃ẋ = −1
2
m′(x)ẋ3, (2.11)
shows that the value E of H is not independent of t. Since Ḣ is cubic in the velocity, equa-
tion (2.11) makes plain that the variable mass system is dissipative [34] (in the quantum case,
the factorization method applied to dissipative systems is associated to complex Riccati equa-
tions [11]). The time derivative (2.11) can be also expressed in canonical form Ḣ = p
m(x)R, with
the reacting thrust R rewritten as
R = R(x, p; t) = R̃(x, ẋ(x, p; t); t) = −m
′(x)
m(x)
(
p2
2m(x)
)
. (2.12)
On the other hand, a simple calculation leads to the phase space form of equation (2.7). We get
ẋ =
∂H
∂p
, ṗ = −∂H
∂x
+R. (2.13)
6 S. Cruz y Cruz and O. Rosas-Ortiz
The differential equations (2.13) correspond to the law of motion of the mechanical position-
dependent mass system in phase space. At time t, their solutions are the canonically conjugate
variables of position and momentum {x, p} = 1, with { , } the Poisson bracket
{f, g} = ∂f
∂x
∂g
∂p
− ∂f
∂p
∂g
∂x
.
Hence, the Hamilton’s equations (2.13) in the Poisson bracket formulation read
ẋ = {x,H}, ṗ = {p,H}+R. (2.14)
In general, the explicit form of m(x) and V(x) determine the Hamiltonian’s domain of defini-
tion D(H). A point transformation is then expected to reduce either equations (2.13) or (2.14)
to the ones associated with an equivalent constant mass system in the appropriate domain (see
Section 2.4).
To summarize our first results, let us rewrite the Lagrangian and the Hamiltonian associated
to the Newton’s equation of motion (2.1), we have
L =
1
2
m(x)ẋ2 − V(x), H =
p2
2m(x)
+ V(x). (2.15)
These results show that the standard expressions for L and H remain valid if the mass is an
explicit function of the position. Thus, a practical description of the position-dependent mass
particle interacting with an external environment consists of replacing the (constant) massm0 by
the appropriate function of the position m(x) in the conventional expressions L0 =
m0
2 ẋ
2−V(x)
and H0 =
p2
2m0
+V(x). The price to pay is, however, that the Euler–Lagrange and the Hamilton’s
equations are not expressed in the standard form.
2.2.1 Energy constant of motion
As we have seen, the systems with position-dependent mass are describable by either a La-
grangian or a Hamiltonian function. However, the Hamiltonian H = p2
2m(x)+V(x), obtained from
the replacing of the constant mass m0 by a function of the position m(x) in H0 =
p2
2m0
+V(x), is
not time-independent (see equation (2.11)). In this section we are going to construct a constant
of motion for such a system. As the equation (2.13) is equivalent to the nonlinear system (2.2),
we first look for an invariant in the (x, v)-plane representation. For our variable mass system,
a function I of x and v is a constant of motion whenever the following equation is true
d
dt
I = ẋ
∂I
∂x
+ v̇
∂I
∂v
= v
∂I
∂x
+
[
F
m
− m′
m
v2
]
∂I
∂v
= 0.
Here we have used (2.2). After multiplying by φ(x, v), a function on (x, v) to be determined,
one gets
∂I
∂x
=
[
φ
m′
m
]
v2 − F
m
φ, (2.16)
∂I
∂v
= vφ. (2.17)
Note that φ = αmβ, with α and β constants, allows to write φm
′
m = 1
β
∂φ
∂x . Therefore (2.16) can
be expressed as
∂I
∂x
=
v2
β
∂φ
∂x
− F
m
φ =
∂
∂x
[
v2
β
φ−
∫ x F
m
φdr
]
. (2.18)
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 7
The introduction of (2.18) in (2.17) gives ∂I
∂v = 2
β vφ. Then we make β = 2 to get
I = α
[
m2v2
2
+
∫ x
m
(
∂V
∂r
)
dr
]
. (2.19)
Taking α = m−10 with m0 in mass units we see that (2.19) is a constant of motion expressed in
energy units, we write
H(x,v) =
m2v2
2m0
+
∫ x m
m0
(
∂V
∂r
)
dr. (2.20)
Thus, the invariant H(x,v) represents the conservative Hamiltonian of our variable mass system
in the (x, v)-plane. (Note even that this is reduced to a constant mass Hamiltonian in the case
m = m0.) The statement is refined using integration by parts
H(x,v) =
m
m0
H̃ ≡ m
m0
[
p2
2m
+ Ṽ
]
, (2.21)
where
Ṽ = V − m0
m
∫ x( m
m0
)′
Vdr
is a modification of the potential V in the phase space representation. As we can see, the
Hamiltonians H(x,v) and H̃ are equivalent up to the multiplicative function m
m0
. Clearly H̃ is
the variable mass Hamiltonian H plus a function of the position defined by the potential V.
As H(x,v) is a constant of motion, equation (2.21) makes plain what is missing in H to be
time-independent. Moreover, solving (2.21) for H yields
H =
m0
m
[
H(x,v) +
∫ x( m
m0
)′
Vdr
]
=
p2
2m
+ V,
which is consistent with the defintion of H given in (2.10).
Let us take full advantage of the invariant (2.20). For this, note that the first additive term
in (2.20) can be expressed in momentum-like form by using
p := mv, m :=
m2
m0
.
Therefore
H =
p2
2m
+ Veff(x), Veff(x) =
∫ x m
m0
(
∂V
∂r
)
dr, (2.22)
is the HamiltonianH(x,v) in the (x,p)-plane representation. Concerning the equations of motion,
the straightforward calculation leads to
ẋ =
∂H
∂p
, −ṗ =
∂H
∂x
, (2.23)
where we have used (2.14), (2.12) and (2.2). Then, the time-variation of f , an arbitrary function
of x, p, and t, is ruled by
d
dt
f = {f,H}x,p +
∂f
∂t
, (2.24)
8 S. Cruz y Cruz and O. Rosas-Ortiz
where
{f, g}x,p =
∂f
∂x
∂g
∂p
− ∂f
∂p
∂g
∂x
. (2.25)
In particular, if f=H then d
dtH=0. On the other hand, using (2.25) we realize that {x,p}x,p=1,
so that x and p are conjugate variables. The equations of motion (2.23) can be now expressed as
ẋ = {x,H}x,p, ṗ = {p, H}x,p. (2.26)
For the sake of simplicity, hereafter we shall omit the sublabels in the brackets {·, ·}x,p.
At this stage some words on the differences between the dynamics of the Hamiltonian H
and that of H are necessary. In the former case, for a variable mass m(x) subjected to a given
potential V(x), the Hamiltonian in standard form H = p2
2m(x) +V(x) involves the modification of
the dynamical equations from ẋ = ∂H0
∂p and ṗ = −∂H0
∂x to those given in (2.13). Hence, the simple
substitution m0 → m(x) in H0 =
p2
2m0
+V(x) to get H makes the problem more involved. In the
second case, the invariantH is the Hamiltonian of a variable mass m = m2/m0 which is subjected
to the effective potential Veff(x) rather than being acted by the potential V(x). In this latter
picture, the dynamical laws are of the standard form (see equation (2.26)). Thus, to preserve
the form of the canonical equations, the mass m as well as the potential V must be transformed
into m and Veff respectively. On the other hand, to preserve the form of the Hamiltonian after
making m0 → m(x), the dynamical equations must be modified according to (2.13).
2.2.2 First applications
To close this section let us stress that, although the Lagrangian and Hamiltonian functions are
not unique, our expressions (2.15) are ensured by the conditions for the existence of standard
Lagrangians for equations with space-dependent coefficients discussed in [34]. Let us multiply
equation (2.1) by 1/m(x), after the identification
b(x) =
m′(x)
m(x)
=
d lnm(x)
dx
, c(x) =
1
m(x)
, g(x) =
dV(x)
dx
,
we arrive at
ẍ+ b(x)ẋ2 + c(x)g(x) = 0.
So that this last equation corresponds to a dissipative system of variable mass and admits
a Lagrangian description (see [34, Proposition 3]). The related standard Lagrangian is
L(x, ẋ) =
1
2
ẋ2eIb(x) −
∫ x
c(x̃)g(x̃)eIb(x̃)dx̃, (2.27)
where the quantity
Ib(x) =
∫ x
b(x̃)dx̃ = lnm(x)
reduces (2.27) to our Lagrangian (2.15). This standard form of writing L and H is appropriate
to recover (as particular cases) some of the Lagrangians and Hamiltonians already reported
in a different context by other authors. For example, consider a mass-function me(x) and
a potential function V(x) such that
me(x) = m0e
κx/2,
dV(x)
dx
= eκx/2
dV (x)
dx
, (2.28)
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 9
with m0 and 1/κ constants expressed in mass and position units respectively. The Newton’s
equation (2.5), the Lagrangian and the Hamiltonian (2.15) become
m0ẍ = −dV (x)
dx
− 1
2
m0κẋ
2,
L =
1
2
m0ẋ
2eκx/2 −
∫ x
0
ds eκs/2
dV (s)
ds
, H =
p2
2m0
e−κx/2 +
∫ x
0
ds eκs/2
dV (s)
ds
.
On the other hand, if we now take
me(x) = m0e
−κx/4 and
dV(x)
dx
= e3κx/4
dV (x)
dx
the Hamiltonian (2.22) becomes
H =
p2
2m0
eκx/2 +
∫ x
ds eκs/2
dV (s)
ds
.
These last expressions are in agreement with those reported in [43] for a system of constant
massm0, subjected to a ‘force quadratic in the velocity’−1
2m0κẋ
2. An immediate generalization,
considering now a mass-functionm(x) = m0 exp
(
1
2κ(x)
)
, can be put in connection to the system
of constant mass discussed in [7]. It is remarkable that the results for constant masses reported
in [7, 43] are also derivable for a mass that varies exponentially with the position, a situation that
seems to be unnoticed in such references. Moreover, no solutions to the corresponding canonical
problem (2.14) are given in neither [43] nor [7]. We get explicit solutions to this problem in
Section 3.4.
Next, we shall construct solutions to the system (2.14) for m(x) an arbitrary, differentiable
and integrable function of x, and the properly chosen potential V(x).
2.3 Factorization and deformed algebras
The dynamical problem (2.26) can be studied in two general ways (compare with the quantum
problems studied in [16]). First, given a specific potential V(x) acting on the mass m(x), the
related phase trajectories are found. Second, given an algebra which rules the dynamical law of
the mass, the potential and phase trajectories are constructed in a purely algebraic form. Our
aim here is to follow the second approach. For this we shall extend the factorization method
discussed in [13, 29] to the case of classical systems having a position-dependent mass and
obeying the dynamical law (2.1). As we have discussed in the previous sections, it is better
to face the problem in the (x, p)-plane. The factorization of the Hamiltonian (2.22) leads in
a natural form to the identification of a pair of time-dependent integrals of motion Q± which,
in turn, allows the construction of the phase trajectories (x(t), p(t)) associated to the canonical
equations.
We look for a couple of complex functions A+(x, p; t), A−(x,p; t), and a constant ε such that
the Hamiltonian (2.22) becomes factorized
H = A+A− + ε = A−A+ + ε. (2.29)
The inspection of these last equations suggest to define A± as follows
A± = ∓if(x) p√
2m(x)
+ g(x)
√
γH, (2.30)
with f(x) and g(x) functions to be determined. Here, we are considering the possibility of bound
states (confined motion) for which the Hamiltonian H is negative (H < 0). Thereby, according
10 S. Cruz y Cruz and O. Rosas-Ortiz
to the sign of H, γ will be either +1 or −1 such that
√
γH is real in (2.30). The introduction
of (2.30) into (2.29) leads to the relationships
f2(x) + γg2(x) = 1, (2.31)
ε = Veff(x)f2(x). (2.32)
Given m(x) and Veff(x), the functions A± and the Hamiltonian H induce a Poisson structure,
i
{
A−,A+
}
=
2√
2m(x)
W (f, g)
√
γH,{
H,A+
}
= A+
{
A−,A+
}
,
{
H,A−
}
= −
{
A−,A+
}
A−, (2.33)
whereW (f, g) = fg′−f ′g is the Wronskian of f and g, and we have used (2.32). Now, we ask the
system (2.33) to close a deformed Poisson algebra by demanding that {A−,A+} be expressed
in terms of the powers of
√
γH. The easiest way to satisfy this condition is by looking for the
solutions of equation
W (f, g) = α
√
2m(x), α = const. (2.34)
Using (2.31), this last equation is reduced to quadratures
∫
dg√
1− γg2
=
√
2α2m0
∫ x
c
J(t)dt, J(x) =
√
m(x)
m0
. (2.35)
The constants m0 and c are expressed in mass and length units respectively. Considering the
sign of the Hamiltonian H, equation (2.35) gives
g(x) =
sin
[√
2α2m0
∫ x
c
J(t)dt
]
, γ = 1, H > 0,
sinh
[√
2α2m0
∫ x
c
J(t)dt
]
, γ = −1, H < 0.
(2.36)
The expression for f(x) is obtained after the substitution of (2.36) into (2.31). At this stage we
realize that the relationships (2.31) and (2.32) define the potential Veff in terms of the g-function
Veff(x) =
ε
1− γg2(x)
=
ε
cos2
[√
2α2m0
∫ x
c J(t)dt
] , H > 0,
ε
cosh2
[√
2α2m0
∫ x
c J(t)dt
] , H < 0.
(2.37)
Thus, the potential allowing equations (2.34) and (2.35) is not arbitrary. As we can see, given the
mass m(x), a potential of the Pöschl–Teller form (2.37) is such that the system (2.33) becomes
the deformed Poisson algebra defined by
i
{
A−,A+
}
= 2α
√
γH, (2.38)
i
{
H,A±
}
= ±2α
√
γHA±. (2.39)
The latter results are consistent with our factorization approach (2.29), (2.30). Indeed, using
relation (2.39), it is easy to verify the following Poisson brackets
{H,A+A−} = {H,A−A+} = 0.
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 11
Hence, the factorization (2.29) makes sense because the productsA+A− andA−A+ are functions
of the Hamiltonian H. In particular, up to an arbitrary additive constant, each of them can be
chosen to be proportional to H. Moreover, using (2.39) and the Jacobi identity
{H, {A−,A+}}+ {A−, {A+, H}}+ {A+, {H,A−}} = 0,
one arrives at the Poisson bracket
{H, {A−,A+}} = −i2α
{
A+A−,
√
γH
}
= 0.
So that {A−,A+} is also a function of H and (2.38) makes sense.
The relevance of equation (2.39) is clear, it implies both the factorization (2.29) and the
Poisson bracket (2.38). This also allows the construction of first integrals of motion for the
problem we are dealing with. Let us introduce the functions Q± = θ±(t)A±, and calculate their
total time derivative. According to (2.24), we have
dQ±
dt
= {Q±, H}+ ∂Q±
∂t
=
[
±i2α
√
γHθ±(t) +
∂θ±
∂t
]
A±. (2.40)
The θ-functions are determined by canceling the expression in square brackets. For E the value
of H, taken as a parameter in the context of Section 2.1, one gets θ±(t) = θ0e
∓i2α
√
γEt (hereafter
θ0 = 1). These time-dependent functions also produce a null total time derivative (2.40), so
that Q± are the following time-dependent integrals of motion
Q± = exp
[
∓ i2α
√
γEt
]
A±. (2.41)
Remark that Q+ is the complex conjugation of Q−, therefore Q+Q− = |Q−|2 = H − ε. Let q±
be the value of the integral of motion Q±, in polar form this can be written as
q± =
√
E − εe±iφ0 , φ0 ∈ R. (2.42)
Let us stress that E − ε ≥ 0 implies E ≥ ε > 0 for H > 0, and |E| ≤ |ε| for H < 0. The values
of A± can be now obtained from (2.30), (2.41) and (2.42):
∓if(x) p√
2m(x)
+ g(x)
√
γE =
√
E − ε exp
[
± i(φ0 + 2α
√
γEt)
]
.
The latter equations lead to the phase trajectories (x(t), p(t)) of bound states (confined motion)
with energy E greater than or equal to the global minimum of the potential. The constant φ0
must be fixed by the initial conditions. We have
g(x(t)) =
√
E − ε
γE
cos
[
φ0 + 2α
√
γEt
]
, (2.43)
p(t) = −
√
2(E − ε)
π(t)
sin
[
φ0 + 2α
√
γEt
]
,
where
π(t) =
f2(x(t))
m(x(t))
.
The explicit form of x as a function of E and t is obtained from (2.43) and (2.36) by using the
composition [g−1 ◦ g](x(t)).
12 S. Cruz y Cruz and O. Rosas-Ortiz
2.4 Point transformations
In Section 2.3 we have shown that potentials of the Pöschl–Teller form (2.37) give rise to the
deformed Poisson algebras (2.38), (2.39); these last associated to the dynamics of a particle of
mass varying with the position. Now, we want to make a point transformation leading from the
canonical coordinates, x and p, of the variable mass m(x), to the coordinates, Q and P , of an
equivalent system of constant mass m0. As usual, we shall write
Q = Q(x, p; t) and P = P (x, p; t)
for the point transformation we look for. Using (2.13) we arrive at
Q̇ = {Q,H}+R
∂Q
∂p
+
∂Q
∂t
, Ṗ = {P,H}+R
∂P
∂p
+
∂P
∂t
.
We want these equations to have the same form as those given in (2.13) for a function K(Q,P ; t)
to be determined. Thus, we ask for
Q̇ =
∂K
∂P
, Ṗ = −∂K
∂Q
+RK , (2.44)
where RK is the reacting thrust (2.12) expressed in the new coordinates. For the sake of
simplicity, let us assume that Q is not an explicit function of neither p nor t, and that P is
time-independent. We have
∂K
∂P
= {Q,H}, −∂K
∂Q
+RK = {P,H}+R
∂P
∂p
.
Let K be the Hamiltonian function associated to a system of constant mass m0 under the action
of a given potential V (Q),
K =
P 2
2m0
+ V (Q).
One easily identifies that the point transformations
Q =
∫ x
√
m(s)
m0
ds and P =
√
m0
m(x)
p (2.45)
allow the mapping from the variable mass Hamiltonian H to the new one K. It is a matter of
substitution to verify that these new variables satisfy the equations (2.44), explicitly
Q̇ =
P
m0
, Ṗ = −dV (Q)
dQ
+RK .
That is, (2.45) represents the canonical transformation from the dynamics of the position-
dependent mass Pöschl–Teller potential (2.37) to the one of a constant mass system. The
similar approach shows that the mapping (x, p)→ (Q̃, P̃ ), with Q̃ =
∫ x
J(s)ds and P̃ = p/J(x)
leads from H to the constant mass Hamiltonian K̃ = P̃ 2
2m0
+ Ṽeff(Q̃), where Veff(x) = Ṽeff(Q̃(x)).
3 Position-dependent mass Pöschl–Teller potentials
In the previous sections we have shown that potentials of the Pöschl–Teller form (2.37) lead
to the solving of the canonical equations of motion (2.26) in terms of the invariants defined
by the deformed Poisson algebra (2.38), (2.39). Depending on the sign of the Hamiltonian H,
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 13
equation (2.37) includes two general forms of these potentials and, as a consequence, we also
have two different realizations of the algebra (2.38), (2.39). Namely, for a positive Hamiltonian
H > 0, the identification
a± =
1
α
A±, a0 =
1
α
√
H,
makes clear that the Poisson structure (2.38), (2.39) associated to the dynamical algebra of the
trigonometric position-dependent mass Pöschl–Teller potential
Veff(x) =
ε
cos2
[√
2α2m0
∫ x
c J(t)dt
]
corresponds to the su(1, 1) algebra
i
{
a−, a+
}
= 2a0, i
{
a0, a±
}
= ±a±.
In a similar form, for a negative Hamiltonian H < 0, the identification
a± =
1
α
A±, a0 = − 1
α
√
−H,
shows that the dynamics of a position-dependent mass particle subjected to the hyperbolic
Pöschl–Teller potential
Veff(x) =
ε
cosh2
[√
2α2m0
∫ x
c J(t)dt
]
is nothing but the su(2) Poisson algebra
i
{
a−, a+
}
= −2a0, i
{
a0, a±
}
= ±a±.
It is well known that the above algebras are related to the Pöschl–Teller systems of constant
mass. Here, we have shown that this is the case even if the mass is a function of the position. The
set of Pöschl–Teller potentials is relevant because of its diversity of applications. For instance, in
the constant mass quantum case such a set has been enlarged to a wide family of supersymmetric
potentials [12, 20]. Another example can be found in the study of gravity localization and thick
branes of cosmological problems [24, 25]. Approaches similar to the one presented here have been
also applied to get a subalgebra of the Poisson algebra in the constant mass case [23]. There, one
can find transformations mapping the whole phase space of the trigonometric case into a whole
coadjoint orbit of SO(2, 1). The hyperbolic case deserves some caution because, even in the
constant mass case, the positive and negative energies give rise to different Lie groups.
Next, we are going to discuss some specific realizations of our general results by supplying
different forms of the mass-function m(x). In each case, the phase trajectories (x(t),p(t)) are
explicitly derived.
3.1 Doubly singular mass-functions
The mass function
mss(x) = m0
(
1− β(λx)2
1− (λx)2
)
, λ > 0, β < 0, (3.1)
has been used in the study of the inversion potential for NH3 in terms of the density theory [2].
This function is singular at the points x = ±1/λ, and has no zeros in the interval (−1/λ, 1/λ).
In notation of [2], m0 = µ0 should correspond to the reduced mass
µ0 =
3mM
3m+M
,
14 S. Cruz y Cruz and O. Rosas-Ortiz
with m the mass of each of the three hydrogen atoms, and M the mass of the nitrogen atom in
the geometry of a rectangular pyramid where M is in the cusp. The mass-function (3.1) then
corresponds to the situation in which the distance between each pair of masses m is allowed to
change. The parameter 1/λ = r0 stands for the separation between m and M at the planar
equilibrium geometry, x is the inversion coordinate and the negative parameter β is given by
β = −3m/M . Here, m0 > 0 and 1/λ > 0 are arbitrary constants expressed in mass and distance
units respectively. The negative, dimensionless parameter β is also arbitrary.
To get explicit forms of the solutions we use (2.35) and arrive at
∫ x
0
Jss(t)dt =
∫ x
0
√
mss(t)
m0
dt =
1
λ
∫ λx
0
√
1− βt2
1− t2
dt =
1
λ
Eint
(
arcsin(λx), β1/2
)
(hereafter we take c = 0), with Eint(φ, k) the incomplete elliptic integral of the second kind [22]
Eint (φ, k) =
∫ sinφ
0
√
1− (kt)2
1− t2
dt.
Potentials (2.37) are in this case given by
Vss(x) =
ε
cos2
[√
2m0
α
λEint
(
arcsinλx, β1/2
)] , H > 0,
ε
cosh2
[√
2m0
α
λEint
(
arcsinλx, β1/2
)] , H < 0.
The domains of definition for these potentials are
Dss> =
{
x
∣∣∣−π
2
<
√
2m0
α
λ
Eint
(
arcsinλx, β1/2
)
<
π
2
}
,
and Dss< = (−1/λ, 1/λ) respectively. Finally, the corresponding phase trajectories are obtained
from the expressions
sin
√
2m0
α
λ
Eint
(
arcsinλx(t), β1/2
)
=
√
E − ε
E
cos
(
2
√
Eαt+ φ0
)
,
p(t) = −
√
2m0(E − ε) (1− β(λx(t))2)√
1− (λx(t))2 cos
[√
2m0
α
λEint
(
arcsinλx(t), β1/2
)] sin (2√Eαt+ φ0
)
,
and
sinh
√
2m0
α
λ
Eint
(
arcsinλx(t), β1/2
)
=
√
E − ε
−E
cos
(
2
√
−Eαt+ φ0
)
,
p(t) = −
√
2m0(E − ε) (1− β(λx(t))2)√
1− (λx(t))2 cosh
[√
2m0
α
λEint
(
arcsinλx(t), β1/2
)] sin (2√−Eαt+ φ0
)
.
The potentials Vss(x) and phase trajectories (x(t),p(t)) have been depicted in Figs. 1 and 2 for
different values of the parameters. From Fig. 1 (left) we realize that only confined motion is
allowed for the domain Dss>. The massmss(x) is singular at the edges of Dss>, where the particle
reaches the turning points and its momentum p is zero. The minimal amount of mass is obtained
at origin, so that the mass increases in value as the particle approaches the turning points and
conversely, the particle losses mass as it approaches the origin. The momentum p evolves in
time in a quite similar manner: For bounded energies greater than the global minimum of the
potential, the particle acquires a momentum p(t) which is greater as the particle approaches
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 15
Figure 1. Upper line. Position dependent-mass trigonometric Pöschl–Teller potentials for the mass-
functions mss (left), ms (center), mR (right), and the parameters α = 1/2, β = −1, m0 = λ = 1. Lower
line. Some of the related (confined motion) classical trajectories in phase space for φ0 = 0, ε = 2 and
E = 2.5 (red curve), E = 4 (dashed curve), E = 5.5 (blue curve).
one of the turning points. Once there, the momentum of the particle changes in sign so that
the motion is reverted by a strong acceleration. This relationship between the maxima of the
mass and the maxima of the momentum is also found in the confined motion for H < 0 (Fig. 2,
left). There, it is also true that the momentum p is zero at the points where the mass is
divergent. The same can be said for the other masses analyzed in the sequel. Of particular
interest, the regular mass case (discussed in Section 3.3) is such that the momentum p and the
mass-function mR(x) are maxima at the origin, and both of them take their lower admissible
values at the corresponding turning points.
3.2 Singular mass-functions
Given a mass function m(x), the quantum problem for the potential V (x) can be solved by
the mapping of the Schrödinger equation of m(x) to the Schrödinger equation of a constant
mass m0 (for a general discussion see [16]). In the trivial case of a constant mass m0, the new
potential Veff (expressed in the new coordinates) is the same as the former one V . In general,
the mass function
ms(x) =
m0
(1 + λx)2
(3.2)
has been shown to be the simplest nontrivial case in which Veff = V [16]. This mass is also
connected with the revival of wave packets in a position-dependent mass infinite well [40], and
was used in our previous analysis of diverse position-dependent mass oscillators [14, 15, 16, 17].
The function (3.2) is singular at the point x = −1/λ and has no zeros in (−1/λ,+∞). The
16 S. Cruz y Cruz and O. Rosas-Ortiz
Figure 2. Upper line. Position dependent-mass hyperbolic Pöschl–Teller potentials for the mass-
functions mss (left), ms (center), mR (right), and the parameters α = 1/2, β = −1, m0 = λ = 1.
Lower line. Some of the related (confined motion) classical trajectories in phase space for φ0 = 0, ε = −2
and E = −1.5 (red curve), E = −1 (dashed curve), E = −0.5 (blue curve).
potentials (2.37) read as
Vs(x) =
ε
cos2
[√
2m0
α
λ ln (1 + λx)
] , H > 0,
ε
cosh2
[√
2m0
α
λ ln (1 + λx)
] , H < 0,
with the domains of definition
Ds> =
{
x
∣∣∣−π
2
<
√
2m0
α
λ
ln (1 + λx) <
π
2
}
,
and Ds< = [−1/λ,∞) respectively. The corresponding phase trajectories are ruled by the
expressions
x(t) =
1
λ
[
exp
(
λ
α
√
2m0
arcsin
[√
E − ε
E
cos
(
2
√
Eαt+ φ0
)])
− 1
]
, H > 0,
1
λ
[
exp
(
λ
α
√
2m0
arcsinh
[√
E − ε
−E
cos
(
2
√
−Eαt+ φ0
)])
− 1
]
, H < 0,
and
p(t) =
−
√
2m0 (E − ε) sin
(
2
√
Eαt+ φ0
)
(1 + λx(t)) cos
[√
2m0
α
λ ln (1 + λx(t))
] , H > 0,
−
√
2m0 (E − ε) sin
(
2
√
−Eαt+ φ0
)
(1 + λx(t)) cosh
[√
2m0
α
λ ln (1 + λx(t))
] , H < 0.
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 17
The potentials Vs(x) and phase trajectories (x(t),p(t)) have been depicted in Figs. 1 and 2 for
different values of the parameters. The description of the behavior of p(t) and m(x) is similar
to that given in Section 3.1.
3.3 Regular mass-functions
The mass function
mR(x) =
m0
1 + (λx)2
appears in the study of diverse oscillators including the nonlinear one [8, 10], and the related
position-dependent mass versions [14, 15, 16]. This is a regular function defined on the whole
real line. The Pöschl–Teller potentials
VR(x) =
ε
cos2
[√
2m0
α
λ arcsinhλx
] , H > 0,
ε
cosh2
[√
2m0
α
λ arcsinhλx
] , H < 0
are defined in
DR> =
{
x
∣∣∣−π
2
<
√
2m0
α
λ
arcsinhλx <
π
2
}
,
and DR< = R respectively. The phase trajectories read as
x(t) =
1
λ
sinh
[
λ
α
√
2m0
arcsin
(√
E − ε
E
cos
(
2
√
Eαt+ φ0
))]
, H > 0,
1
λ
sinh
[
λ
α
√
2m0
arcsinh
(√
E − ε
−E
cos
(
2
√
−Eαt+ φ0
))]
, H > 0,
and
p(t) =
−
√
2m0 (E − ε)
1 + (λx(t))2
sin
(
2
√
Eαt+ φ0
)
cos
[
α
λ
√
2m0 arcsinh (λx(t))
] , H > 0,
−
√
2m0 (E − ε)
1 + (λx(t))2
sin
(
2
√
−Eαt+ φ0
)
cosh
[
α
λ
√
2m0 arcsinh (λx(t))
] , H < 0.
The potentials VR(x) and phase trajectories (x(t),p(t)) have been depicted in Figs. 1 and 2 for
different values of the parameters. The description of the behavior of p(t) and m(x) is similar
to that given in Section 3.1.
3.4 Exponential mass-functions
The exponential mass-functionme(x), introduced in equation (2.28), is a regular function defined
on the whole real line. Here, we shall take κ < 0 in order to get finite masses in the positive
regime of the domain. The domains of definition of the Pöschl–Teller potentials
Ve(x) =
ε
cos2
[√
2α2m0
4
κ
(
eκx/4 − 1
)] , H > 0,
ε
cosh2
[√
2α2m0
4
κ
(
eκx/4 − 1
)] , H < 0
(3.3)
18 S. Cruz y Cruz and O. Rosas-Ortiz
Figure 3. Upper line. Position dependent-mass Pöschl–Teller potentials for the exponential mass-
function me(x) in the domains of definition D1
e> (left), D2
e> (center), and De< (right). The parameters
are α = 1, m0 = 1/2, and ε = 2, κ = −2 (left), ε = 2, κ = −4 (center), ε = −2, κ = −4 (right).
Lower line. Some of the related (confined motion) classical trajectories in phase space for φ0 = 0 and
E = 2.5, 4, 5.5 (left and center), E = −1.75,−1.5,−1 (right).
are respectively given by
{
D1
e> = [x−0 , x
+
0 ], for any κ < 0, m0 > 0, R 3 α 6= 0,
D2
e> = [x−0 ,+∞), with 8
√
2α2m0 ≤ |κ|π,
and De< = R, with
x±0 = − 4
|κ|
ln
[
1∓ π|κ|
8
√
2α2m0
]
.
Potential Ve(x) in (3.3) is singular at x = x±0 for the domain D1
e>. This is also singular at
x = x±0 , and acquires a finite value as x → +∞ in the domain D2
e>. Finally, this potential
cancels as x→ −∞, and goes to a finite value as x→ +∞ in the domain De< (see Fig. 3). The
related phase trajectories are constructed according to
x(t) =
4
κ
ln
[
κ
4
√
2α2m0
arcsin
(√
E − ε
E
cos
(
φ0 + 2α
√
Et
))
+ 1
]
, H > 0,
4
κ
ln
[
κ
4
√
2α2m0
arcsinh
(√
E − ε
−E
cos
(
φ0 + 2α
√
−Et
))
+ 1
]
, H < 0,
(3.4)
Dynamical Equations of Mechanical Systems with Position-Dependent Mass 19
and
p(t) =
−
√
2(E − ε)m0e
κx/4 sin
(
φ0 + 2α
√
Et
)
cos
[√
2α2m0
4
κ
(
eκx/4 − 1
)] , H > 0,
−
√
2(E − ε)m0e
κx/4 sin
(
φ0 + 2α
√
−Et
)
cosh
[√
2α2m0
4
κ
(
eκx/4 − 1
)] , H < 0,
(3.5)
The potentials Ve(x) and phase trajectories (x(t),p(t)) have been depicted in Fig. 3 for different
values of the parameters. In this case we can distinguish three general kinds of potential: I. The
potential is such that only confined motion is allowed (Fig. 3, left). II. The potential Ve(x) is
such that scattering and positive bounded states (confined motion) are allowed (Fig. 3, center).
III. Negative and positive energies can be associated to scattering states, and negative bounded
states are allowed (Fig. 3, right). The description of the behavior of p(t) and m(x) is similar to
that given in Section 3.1. With the trajectories in the phase space (3.4) and (3.5) we provide an
explicit solution to the canonical problem involving “forces quadratic in the velocity” discussed
in [43] and [7].
4 Conclusions
We have constructed the Lagrangian for a particle suffering a spatial variation of its mass. The
corresponding Euler–Lagrange equations have been shown to recover the Newton’s dynamical
law associated to this system. As a consequence of the position-dependence of the mass, there
is a force quadratic in the velocity which is non-inertial and represents the thrust of the system.
The Lagrangian for this dissipative system is of the standard form L = p2
2m(x) − V(x), in corre-
spondence with the conditions studied in [34]. The construction of the related Hamiltonian also
leads to the standard form H = p2
2m(x) +V(x). Thus, we have found that a simple description of
this system starts by the replacing of the (constant) mass m0 by the appropriate function of the
position m(x) in the conventional expressions of L and H. Accordingly, it has been shown that
the canonical equations of motion must contain a term including the thrust in order to recover the
Newton’s equation. Since H is not time-independent, we have explicitly constructed an energy
constant of motion H leading to dynamical equations that have the form of the Hamilton ones.
A canonical transformation mapping the variable mass problem to the one of a constant mass
has been also identified. Departing from the factorization of the Hamiltonian H = A+A−+ε, we
have obtained two time-dependent integrals of motion Q± which, in turn, allow the construction
of the trajectories in the phase space (x(t), p(t)). Such invariants are associated to potential func-
tions which are necessarily of the Pöschl–Teller form if one looks for the related spectrum gener-
ating algebras. The latter are obtained from the Poisson structure defined by the factorization
of the Hamiltonian and demanding that the Poisson brackets of A± and H be expressed in terms
of the polynomials of H. Two general forms of the Pöschl–Teller potentials have been found, one
is of the trigonometric type and is connected with the su(1, 1) Lie algebra, the other is of hyper-
bolic type and is associated to the su(2) algebra. Different solutions for the canonical equations
of motion have been explicitly given in terms of specific forms of the mass-function m(x). As
particular cases, the Lagrangians and Hamiltonians already reported by other authors [7, 43]
have been recovered by the appropriate selection of the mass function m(x). Moreover, in con-
tradistinction with such works, we provide explicit solutions to the corresponding equations of
motion. We stress that the singular oscillator, generalized Pöschl–Teller and the Morse poten-
tials can be also included in our approach but they require factorizing functions A± different
from the ones used in the present work. Indeed, the underlying Poisson structures of these last
systems do not always give rise to a Lie algebra (see for instance [29]). Finally, we mention that
20 S. Cruz y Cruz and O. Rosas-Ortiz
similar approaches to that presented here have been applied in the analysis of the constant mass
versions of the Pöschl–Teller and Morse potentials for classical and quantum dynamics [1, 23].
Acknowledgements
The authors thank the anonymous referees for their comments to improve the presentation
and motivation of the paper. The financial support of CONACyT-Mexico (project 152574),
MICINN-Spain (project MTM2009-10751), IPN grant COFAA and projects SIP20120451, SIP-
SNIC-2011/04, is acknowledged.
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1 Introduction
2 Deformed algebras of a particle with position dependent mass
2.1 Newtonian framework
2.2 Canonical framework
2.2.1 Energy constant of motion
2.2.2 First applications
2.3 Factorization and deformed algebras
2.4 Point transformations
3 Position-dependent mass Pöschl-Teller potentials
3.1 Doubly singular mass-functions
3.2 Singular mass-functions
3.3 Regular mass-functions
3.4 Exponential mass-functions
4 Conclusions
References
|
| id | nasplib_isofts_kiev_ua-123456789-149209 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1815-0659 |
| language | English |
| last_indexed | 2025-12-02T04:27:21Z |
| publishDate | 2013 |
| publisher | Інститут математики НАН України |
| record_format | dspace |
| spelling | Sara Cruz y Cruz Rosas-Ortiz, Oscar 2019-02-19T18:43:27Z 2019-02-19T18:43:27Z 2013 Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass / Sara Cruz y Cruz, Oscar Rosas-Ortiz // Symmetry, Integrability and Geometry: Methods and Applications. — 2013. — Т. 9. — Бібліогр.: 46 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 35Q99; 37J99; 70H03; 70H05 DOI: http://dx.doi.org/10.3842/SIGMA.2013.004 https://nasplib.isofts.kiev.ua/handle/123456789/149209 We analyze the dynamical equations obeyed by a classical system with position-dependent mass. It is shown that there is a non-conservative force quadratic in the velocity associated to the variable mass. We construct the Lagrangian and the Hamiltonian for this system and find the modifications required in the Euler-Lagrange and Hamilton's equations to reproduce the appropriate Newton's dynamical law. Since the Hamiltonian is not time invariant, we get a constant of motion suited to write the dynamical equations in the form of the Hamilton's ones. The time-dependent first integrals of motion are then obtained from the factorization of such a constant. A canonical transformation is found to map the variable mass equations to those of a constant mass. As particular cases, we recover some recent results for which the dependence of the mass on the position was already unnoticed, and find new solvable potentials of the Pöschl-Teller form which seem to be new. The latter are associated to either the su(1,1) or the su(2) Lie algebras depending on the sign of the Hamiltonian. This paper is a contribution to the Special Issue “Superintegrability, Exact Solvability, and Special Functions”. The full collection is available at http://www.emis.de/journals/SIGMA/SESSF2012.html. The authors thank the anonymous referees for their comments to improve the presentation and motivation of the paper. The financial support of CONACyT-Mexico (project 152574), MICINN-Spain (project MTM2009-10751), IPN grant COFAA and projects SIP20120451, SIPSNIC-2011/04, is acknowledged. en Інститут математики НАН України Symmetry, Integrability and Geometry: Methods and Applications Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass Article published earlier |
| spellingShingle | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass Sara Cruz y Cruz Rosas-Ortiz, Oscar |
| title | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass |
| title_full | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass |
| title_fullStr | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass |
| title_full_unstemmed | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass |
| title_short | Dynamical Equations, Invariants and Spectrum Generating Algebras of Mechanical Systems with Position-Dependent Mass |
| title_sort | dynamical equations, invariants and spectrum generating algebras of mechanical systems with position-dependent mass |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/149209 |
| work_keys_str_mv | AT saracruzycruz dynamicalequationsinvariantsandspectrumgeneratingalgebrasofmechanicalsystemswithpositiondependentmass AT rosasortizoscar dynamicalequationsinvariantsandspectrumgeneratingalgebrasofmechanicalsystemswithpositiondependentmass |