Searcharxiv⌕ Search

arXiv subjects

Nivaldo A. Lemos

Publications and source records attributed to Nivaldo A. Lemos.

At least 19 recordsLinked to original sources
arXiv2026

Stationary states for a particle in a box with slanted walls

The quantum mechanical problem of a particle in a infinite potential well with slanted walls is studied. The energy eigenvalues are determined by a transcendental equation involving the Airy function of the first kind. Two limiting cases are discussed. Next, the allowed energies are found by graphical and numerical methods. The graphical analysis hints that for large quantum numbers the consecutive energy levels get arbitrarily close together. This asymptotic behavior of the energy spectrum actually holds, as shown by means of an intuitive argument and confirmed by resorting to basic properties of Airy functions. In a sense, Bohr's correspondence principle is more accurately fulfilled than in the cases of the standard particle in a box or the harmonic oscillator. A state is regarded the more confined the less the probability of finding the particle in the classically forbidden region. For some stationary states the confinement degree is calculated and a comparison is made with the harmonic oscillator, for which the ground state is the least confined of all stationary states. In the symmetric case --- right and left walls of equal slope --- something unexpected happens: beyond a certain critical slope the ground state is no longer the least confined eigenstate. Over and above its interesting physical features, this problem provides students with the opportunity to get acquainted with Airy functions, which do not belong to the standard mathematical repertoire of physics undergraduates.

quant-ph↗
arXiv2026

Bound states in a semi-infinite square potential well

The finite square potential well is a staple problem in introductory quantum mechanics. There is an extensive literature on the determination of the allowed energies, which requires the solution of a transcendental equation by numerical, graphical or approximate analytic methods. Here we investigate the less explored problem of a particle in a semi-infinite potential well. The energy eigenvalues, which are also determined by a transcendental equation, are found by a standard graphical method, and a simple rule that yields the number of stationary states is provided. Next a simplification of the aforementioned transcendental equation is attempted. During the process pitfalls are encountered and a purportedly simpler graphical treatment of the problem given in the solutions manual to a fine textbook is shown to be flawed. A more careful analysis brings forth the correct simplification, which is shown to be particularly suitable for finding highly accurate approximations to the energy levels. Finally, a class of exact solutions is produced, the associated normalized eigenfunctions are constructed and the probability of finding the particle inside the well is computed.

quant-ph↗
arXiv2025

Are Hilbert Spaces Unphysical? Hardly, My Dear!

It is widely accepted that the states of any quantum system are vectors in a Hilbert space. Not everyone agrees, however. The recent paper ``The unphysicality of Hilbert spaces'' by Carcassi, Calderón and Aidala is a thoughtful dissection of the mathematical structure of quantum mechanics that seeks to pinpoint supposedly unsurmountable difficulties inherent in postulating that the physical states are elements of a Hilbert space. Its pivotal charge against Hilbert spaces is that by a change of variables, which is a change-of-basis unitary transformation, one ``can map states with finite expectation values to those with infinite ones''. In the present work it is shown that this statement is incorrect and the source of the error is spotted. In consequence, the purported example of a time evolution that makes ``the expectation value oscillate from finite to infinite in finite time" is also faulty, and the assertion that Hilbert spaces ``turn a potential infinity into an actual infinity'' is unsubstantiated. Two other objections to Hilbert spaces on physical grounds, both technically correct, are the isomorphism of separable Hilbert spaces and the unavoidable existence of infinite-expectation-value states. The former turns out to be quite irrrelevant but the latter remains an issue without a fully satisfactory solution, although the evidence so far is that it is physically innocuous. All in all, while the authors' thesis that Hilbert spaces must be given up deserves some attention, it is a long way from being persuasive as it is founded chiefly on a misconception and, subsidiarily, on immaterial or flimsy arguments.

quant-ph↗
arXiv2024

Free fall in modified symmetric teleparallel gravity

The status of the equivalence principle in modified symmetric teleparallel gravity is examined. In this theory, minimum length geodesics are distinct from autoparallel geodesics, that is, the ``shortest'' paths are not the ``straightest'' paths. We show that a standard argument that singles out metric geodesics in general relativity does not apply in modified symmetric teleparallel gravity. This is because the latter theory does not obey the equivalence principle in the sense of Weinberg. We argue, however, that the structure of the theory makes it inevitable that a freely falling test particle follows a shortest path, a geodesic of the metric. The geodesic equation that governs the motion of a freely falling test particle involves the Levi-Civita connection, not some other connection obtained by solving the connection field equations of the theory. This also has bearing on whether, under appropriate conditions, modified symmetric teleparallel gravity is fully equivalent to general relativity.

gr-qc↗
arXiv2021

Breakdown of the connection between symmetries and conservation laws for semiholonomic systems

Integrable velocity-dependent constraints are said to be semiholonomic. For good reasons, holonomic and semiholonomic constraints are thought to be indistinguishable in Lagrangian mechanics. This well-founded belief notwithstanding, here we show by means of an example and a broad analysis that the connection between symmetries and conservation laws, which holds for holonomic systems, is not valid in general for systems subject to semiholonomic constraints.

physics.gen-ph↗
arXiv2021

On the consistency of the Lagrange multiplier method in classical mechanics

Problems involving rolling without slipping or no sideways skidding, to name a few, introduce velocity-dependent constraints that can be efficiently treated by the method of Lagrange multipliers in the Lagrangian formulation of the classical equations of motion. In doing so one finds, as a bonus, the constraint forces, which must be independent of the solution of the equations of motion, and can only depend on the generalized coordinates and velocities, as well as time. In this paper we establish the conditions the Lagrangian should obey that guarantee that the constraint forces can be obtained consistently.

physics.class-ph↗
arXiv2020

Complete inequivalence of nonholonomic and vakonomic mechanics: rolling coin on an inclined plane

Vakonomic mechanics has been proposed as a possible description of the dynamics of systems subject to nonholonomic constraints. The aim of the present work is to show that for an important physical system the motion brought about by vakonomic mechanics is completely inequivalent to the one derived from nonholonomic mechanics, which relies on the standard method of Lagrange multipliers in the d'Alembert-Lagrange formulation of the classical equations of motion. For the rolling coin on an inclined plane, it is proved that no nontrivial solution to the equations of motion of nonholonomic mechanics can be obtained in the framework of vakonomic mechanics. This completes previous investigations that managed to show only that, for certain mechanical systems, some but not necessarily all nonholonomic motions are beyond the reach of vakonomic mechanics. Furthermore, it is argued that a simple qualitative experiment that anyone can perform at home supports the predictions of nonholonomic mechanics.

physics.gen-ph↗
arXiv2020

Generalization of the least uncomfortable journey problem

The variational problem of the least uncomfortable journey between two locations on a straight line is simplified by a choice of the dependent variable. It is shown that taking the position, instead of the velocity, as the optimal function of time to be determined does away with the isoperimetric constraint. The same results as those found with the velocity as the dependent variable are obtained in a simpler and more concise way. Next the problem is generalized for motion on an arbitrary curve. In the case of acceleration-induced discomfort, it is shown that, as expected, motion on a curved path is always more uncomfortable than motion on a straight line. It is not clear that this is necessarily the case for jerk-induced discomfort, which appears to indicate that the acceleration provides a more reasonable measure of the discomfort than the jerk. The example of motion on a circular path is studied. Although we have been unable to solve the problem analytically, approximate solutions have been constructed by means of trial functions and the exact solution has been found numerically for some choices of the relevant parameters.

physics.gen-ph↗
arXiv2019

On the least uncomfortable journey from A to B

The problem of the "least uncomfortable journey" between two locations on a straight line, originally discussed by Anderson {\it et al.} (2016 {\it Am. J. Phys.} {\bf 84} 6905) is revisited. When the integral of the square of the acceleration is used as a measure of the discomfort, the problem is shown to be easily solvable by taking the time, instead of the position, as the independent variable. The solution is quite simple and avoids not only complicated differential equations and the computation of cumbersome integrals, but also the inversion of functions by solving cubic equations. Next, the same problem, but now with the integral of the square of the jerk as a measure of the discomfort, is also exactly solved with time as the independent variable and the appropriate boundary conditions, which are derived. It is argued that the boundary conditions imposed on the velocity in Anderson {\it et al.} (2016 {\it Am. J. Phys.} {\bf 84} 6905) are inappropriate not only because they are not always physically realizable but also because they do not lead to the minimum discomfort possible.

physics.gen-ph↗
arXiv2017

Atwood's machine with a massive string

The dynamics of Atwood's machine with a string of significant mass are described by the Lagrangian formalism, providing an eloquent example of how the Lagrangian approach is a great deal simpler and so much more expedient than the Newtonian treatment.

physics.gen-ph↗
arXiv2017

Force and torque of a string on a pulley

Every university introductory physics course considers the problem of Atwood's machine taking into account the mass of the pulley. In the usual treatment the tensions at the two ends of the string are offhandedly taken to act on the pulley and be responsible for its rotation. However such a free-body diagram of the forces on the pulley is not {\it a priori} justified, inducing students to construct wrong hypotheses such as that the string transfers its tension to the pulley or that some symmetry is in operation. We reexamine this problem by integrating the contact forces between each element of the string and the pulley and show that although the pulley does behave as if the tensions were acting on it, this comes only as the end result of a detailed analysis. We also address the question of how much friction is needed to prevent the string from slipping over the pulley. Finally, we deal with the case in which the string is on the verge of sliding and show that this will never happen unless certain conditions are met by the coefficient of friction and the masses involved.

physics.class-ph↗
arXiv2013

Incompleteness of the Hamilton-Jacobi theory

The problem of the motion of a charged particle in an electric dipole field is used to illustrate that the Hamilton-Jacobi method does not necessarily give all solutions to the equations of motion of a mechanical system. The mathematical reason for this phenomenon is discussed. In the particular case under consideration, it is shown how to circumvent the difficulty and find the missing solutions by means of a very special limiting procedure.

physics.gen-ph↗
arXiv2013

On what does not expand in an expanding universe: a very simple model

As the separation between galaxies increases owing to the expansion of the universe, galaxies themselves and smaller bound structures do not grow. An accurate description of the dynamics of cosmic structures requires the full apparatus of general relativity. In order to gain a fairly satisfactory understanding of what does not expand in an expanding universe, however, it suffices to take the harmonic oscillator as prototype of a bound system. More precisely, we show that a study of the quantum dynamics of a nonrelativistic harmonic oscillator in an expanding universe makes it clear that most bound systems do not take part in the overall cosmic expansion. The analysis is elementary and indicates that whether a bound structure partakes in the expansion partially or not at all is essentially determined by a characteristic time scale associated with it.

gr-qc↗
arXiv2008

Can Quantum Gravitational Effects Influence the Entire History of the Universe?

In this work, a flat Friedmann-Robertson-Walker (FRW) universe with dust and a cosmological constant is quantized. By means of a canonical transformation, the classical Hamiltonian is reduced to that of either a harmonic oscillator or anti-oscillator, depending on whether $Λ<0$ or $Λ>0$, respectively. In this way exact solutions to the Wheeler-DeWitt equation can easily be obtained. It turns out that a positive cosmological constant alone may account for an early inflationary regime and a later accelerated expansion phase, with a period of decelerated expansion in between. This suggests that quantum gravitational effects can influence most of the history or even the entire history of the Universe.

gr-qc↗
arXiv2008

Failure of intuition in elementary rigid body dynamics

Suppose a projectile collides perpendicularly with a stationary rigid rod on a smooth horizontal table. We show that, contrary to what one naturally expects, it is not always the case that the rod acquires maximum angular velocity when struck at an extremity. The treatment is intended for first year university students of Physics or Engineering, and could form the basis of a tutorial discussion of conservation laws in rigid body dynamics.

physics.ed-ph↗
arXiv2007

Does the Big Rip survive quantization?

It is known that certain quantum cosmological models present quantum behavior for large scale factors. Since quantization can suppress past singularities, it is natural to inquire whether quantum effects can prevent future singularities. To this end, a Friedmann-Robertson-Walker quantum cosmological model dominated by a phantom energy fluid is investigated. The classical model displays accelerated expansion ending in a Big Rip. The quantization is performed in three different ways, which turn out to lead to the same result, namely there is a possibility that quantum gravitational effects could not remove the Big Rip.

gr-qc↗
arXiv2006

Dynamics of a liquid dielectric attracted by a cylindrical capacitor

The dynamics of a liquid dielectric attracted by a vertical cylindrical capacitor is studied. Contrary to what might be expected from the standard calculation of the force exerted by the capacitor, the motion of the dielectric is different depending on whether the charge or the voltage of the capacitor is held constant. The problem turns out to be an unconventional example of dynamics of a system with variable mass, whose velocity can, in certain circumstances, suffer abrupt changes. Under the hypothesis that the voltage remains constant the motion is described in qualitative and quantitative details, and a very brief qualitative discussion is made of the constant charge case.

physics.ed-ph↗