139 research outputs found
About Gravitomagnetism
The gravitomagnetic field is the force exerted by a moving body on the basis
of the intriguing interplay between geometry and dynamics which is the analog
to the magnetic field of a moving charged body in electromagnetism. The
existence of such a field has been demonstrated based on special relativity
approach and also by special relativity plus the gravitational time dilation
for two different cases, a moving infinite line and a uniformly moving point
mass, respectively. We treat these two approaches when the applied cases are
switched while appropriate key points are employed. Thus, we demonstrate that
the strength of the resulted gravitomagnetic field in the latter approach is
twice the former. Then, we also discuss the full linearized general relativity
and show that it should give the same strength for gravitomagnetic field as the
latter approach. Hence, through an exact analogy with the electrodynamic
equations, we present an argument in order to indicate the best definition
amongst those considered in this issue in the literature. Finally, we
investigate the gravitomagnetic effects and consequences of different
definitions on the geodesic equation including the second order approximation
terms.Comment: 16 pages, a few amendments have been performed and a new section has
been adde
Quantum mechanics and geodesic deviation in the brane world
We investigate the induced geodesic deviation equations in the brane world
models, in which all the matter forces except gravity are confined on the
3-brane. Also, the Newtonian limit of induced geodesic deviation equation is
studied. We show that in the first Randall-Sundrum model the Bohr-Sommerfeld
quantization rule is as a result of consistency between the geodesic and
geodesic deviation equations. This indicates that the path of test particle is
made up of integral multiples of a fundamental Compton-type unit of length
.Comment: 5 pages, no figure
A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 1, level walking
This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
This author accepted manuscript is made available following 24 month embargo from date of publication (August 2017) in accordance with the publisher’s archiving policyThis paper investigates the mechanical response of a modular head-neck interface of hip joint implants under realistic loads of level walking. The realistic loads of the walking activity consist of three dimensional gait forces and the associated frictional moments. These forces and moments were extracted for a 32 mm metal-on-metal bearing couple. A previously reported geometry of a modular CoCr/CoCr head-neck interface with a proximal contact was used for this investigation. An explicit finite element analysis was performed to investigate the interface mechanical responses. To study the level of contribution and also the effect of superposition of the load components, three different scenarios of loading were studied: gait forces only, frictional moments only, and combined gait forces and frictional moments. Stress field, micro-motions, shear stresses and fretting work at the contacting nodes of the interface were analysed. Gait forces only were found to significantly influence the mechanical environment of the head-neck interface by temporarily extending the contacting area (8.43% of initially non-contacting surface nodes temporarily came into contact), and therefore changing the stress field and resultant micro-motions during the gait cycle. The frictional moments only did not cause considerable changes in the mechanical response of the interface (only 0.27% of the non-contacting surface nodes temporarily came into contact). However, when superposed with the gait forces, the mechanical response of the interface, particularly micro-motions and fretting work, changed compared to the forces only case. The normal contact stresses and micro-motions obtained from this realistic load-controlled study were typically in the range of 0-275 MPa and 0-38 µm, respectively. These ranges were found comparable to previous experimental displacement-controlled pin/cylinder-on-disk fretting corrosion studies
A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 2
This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
This author accepted manuscript is made available following 24 month embargo from date of publication (Sept 2017) in accordance with the publisher’s archiving policyA finite element model was developed to investigate the effect of loading regimes caused by various daily activities on the mechanical behaviour of the head-neck taper junction in modular hip replacements. The activities included stair up, stair down, sit to stand, stand to sit, one leg standing and knee bending. To present the real mechanical environment of the junction, in addition to the force components, the frictional moments produced by the frictional sliding of the head and cup were applied to a CoCr/CoCr junction having a 12/14 taper with a proximal mismatch angle of 0.024°. This study revealed that stair up with the highest fretting work per unit of length (1.62 × 104 J/m) was the most critical activity, while knee bending and stand to sit with 1.96 × 103 J/m were the least critical activities. For all the activities, the superolateral region of the neck was identified as the most critical region in terms of having larger values of fretting work per unit of area. This study showed also that the relative micro-motions and contact stresses occurring at the head-neck interface for all the studied activities are mostly in the range of 0–38 µm and 0–350 MPa, respectively. These ranges may be accordingly employed for conducting relevant in-vitro tests to more realistically represent the mechanical environment of taper junctions with the same materials and geometry studied in this work
An analytical calculation of frictional and bending moments at the head-neck interface of hip joint implants during different physiological activities
This study predicts the frictional moments at the head-cup interface and frictional torques and bending moments acting on the head-neck interface of a modular total hip replacement across a range of activities of daily living. The predicted moment and torque profiles are based on the kinematics of four patients and the implant characteristics of a metal-on-metal implant. Depending on the body weight and type of activity, the moments and torques had significant variations in both magnitude and direction over the activity cycles. For the nine investigated activities, the maximum magnitude of the frictional moment ranged from 2.6 to 7.1 Nm. The maximum magnitude of the torque acting on the head-neck interface ranged from 2.3 to 5.7 Nm. The bending moment acting on the head-neck interface varied from 7 to 21.6 Nm. One-leg-standing had the widest range of frictional torque on the head-neck interface (11 Nm) while normal walking had the smallest range (6.1 Nm). The widest range, together with the maximum magnitude of torque, bending moment, and frictional moment, occurred during one-leg-standing of the lightest patient. Most of the simulated activities resulted in frictional torques that were near the previously reported oxide layer depassivation threshold torque. The predicted bending moments were also found at a level believed to contribute to the oxide layer depassivation. The calculated magnitudes and directions of the moments, applied directly to the head-neck taper junction, provide realistic mechanical loading data for in vitro and computational studies on the mechanical behaviour and multi-axial fretting at the head-neck interface.Hamidreza Farhoudi, Reza H. Oskouei, Ali A. Pasha Zanoosi, Claire F. Jones and Mark Taylo
Black holes in the varying speed of light theory
We consider the effect of the \emph{Varying Speed of Light} theory on
non-rotating black holes. We show that in any varying- theory, the
Schwarzschild solution is neither static nor stationary. For a no-charged black
hole, the singularity in the Schwarzschild horizon cannot be removed by
coordinate transformation. Hence, no matter can enter the horizon, and the
interior part of the black hole is separated from the rest of the Universe. If
, then the size of the Schwarzschild radius increases with time. The
higher value of the speed of light in the very early Universe may have caused a
large reduction in the probability of the creation of the primordial black
holes and their population.The same analogy is also considered for the charged
black holes.Comment: 5 page
Boundary Term in Metric f(R) Gravity: Field Equations in the Metric Formalism
The main goal of this paper is to get in a straightforward form the field
equations in metric f(R) gravity, using elementary variational principles and
adding a boundary term in the action, instead of the usual treatment in an
equivalent scalar-tensor approach. We start with a brief review of the
Einstein-Hilbert action, together with the Gibbons-York-Hawking boundary term,
which is mentioned in some literature, but is generally missing. Next we
present in detail the field equations in metric f(R) gravity, including the
discussion about boundaries, and we compare with the Gibbons-York-Hawking term
in General Relativity. We notice that this boundary term is necessary in order
to have a well defined extremal action principle under metric variation.Comment: 12 pages, title changes by referee recommendation. Accepted for
publication in General Relativity and Gravitation. Matches with the accepted
versio
Horizon Problem Remediation via Deformed Phase Space
We investigate the effects of a special kind of dynamical deformation between
the momenta of the scalar field of the Brans-Dicke theory and the scale factor
of the FRW metric. This special choice of deformation includes linearly a
deformation parameter. We trace the deformation footprints in the cosmological
equations of motion when the BD coupling parameter goes to infinity. One class
of the solutions gives a constant scale factor in the late time that confirms
the previous result obtained via another approach in the literature. This
effect can be interpreted as a quantum gravity footprint in the coarse grained
explanation. The another class of the solutions removes the big bang
singularity, and the accelerating expansion region has an infinite temporal
range which overcomes the horizon problem. After this epoch, there is a
graceful exiting by which the universe enters in the radiation dominated era.Comment: 13 pages, 2 figures, to appear in GER
Chameleonic Generalized Brans--Dicke model and late-time acceleration
In this paper we consider Chameleonic Generalized Brans--Dicke Cosmology in
the framework of FRW universes. The bouncing solution and phantom crossing is
investigated for the model. Two independent cosmological tests: Cosmological
Redshift Drift (CRD) and distance modulus are applied to test the model with
the observation.Comment: 20 pages, 15 figures, to be published in Astrophys. Space Sci. (2011
Dark energy from conformal symmetry breaking
The breakdown of conformal symmetry in a conformally invariant scalar-tensor
gravitational model is revisited in the cosmological context. Although the old
scenario of conformal symmetry breaking in cosmology containing scalar field
has already been used in many earlier works, it seems that no special attention
has been paid for the investigation on the possible connection between the
breakdown of conformal symmetry and the existence of dark energy. In this
paper, it is shown that the old scenario of conformal symmetry breaking in
cosmology, if properly interpreted, not only has a potential ability to
describe the origin of dark energy as a symmetry breaking effect, but also may
resolve the coincidence problem.Comment: 11 pages, minor revision, published online in EPJ
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