Accélération

Résumé

L'accélération est une grandeur physique vectorielle, appelée de façon plus précise « vecteur accélération », utilisée en cinématique pour représenter la modification affectant la vitesse d'un mouvement en fonction du temps. La norme (l'intensité) de ce vecteur est appelée simplement « accélération » sans autre qualificatif. Dans le langage courant, l'accélération s'oppose à la décélération (mathématiquement, une accélération négative) et indique l'augmentation de la vitesse ou de la fréquence d'évolution d'un processus quelconque, par exemple l'accélération de la fréquence cardiaque ou celle d'une suite de situations. Approche intuitive vignette|400px|Illustrations de la notion d'accélération :

Le mouvement ne subit pas d'accélération.
La vitesse augmente régulièrement.
La vitesse diminue régulièrement.
L'accélération décrit une courbure de la trajectoire.

De même que la vitesse décrit la modification de la position d'un objet au cours du te

Source officielle

https://fr.wikipedia.org/wiki/Acc%C3%A9l%C3%A9ration

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Publications associées (75)

Collision-Free Intersection Crossing of Mobile Robots Using Decentralized Navigation Functions on Predefined Paths

Benjamin Fankhauser, Denis Gillet, Laleh Makarem

This paper deals with the coordination of a group of mobile robots at an intersection. It focusses on decentralized navigation functions (DNFs) to achieve efficient traffic control. The main challenge is to define virtual potentials, which are used by decentralized navigation functions, such that traffic is both fluent and safe, while taking into account real-world limitations like acceleration, braking and speed limits. Our method consists in defining the navigation function with respect to the desired acceleration profile and is accompanied by a set of visibility conditions that increase the capacity of the intersection in terms of vehicle throughput. Priority conditions have been used to both avoid blockades of robots and to save energy by assigning higher priorities to robots with higher inertias.

2011

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Investigating the effect of isotope substitution on equilibrium and kinetic properties of molecules has become an important tool for estimating the importance of nuclear quantum effects. In this work, we discuss calculating both equilibrium and kinetic isotope effects, i.e., the isotope effects on a system's partition function and a reaction's rate constant. With the help of Feynman's path integral formalism, both quantities can be estimated using standard Monte Carlo methods that scale favorably with system's dimensionality; improving efficiency of such approaches is the main focus of this work. First of all, we developed a novel procedure for changing mass stochastically during an equilibrium isotope effect calculation, and evaluated the numerical benefits of combining it with two popular approaches for calculating isotope effects, using either direct estimators or thermodynamic integration. We demonstrate that the modification improves statistical convergence of both methods, and that it additionally allows to eliminate integration error of thermodynamic integration. The improved methods are tested on equilibrium isotope effects in a model harmonic system and in methane. Then we turn our attention to kinetic isotope effect calculations with the quantum instanton approximation, a method whose path integral implementation belongs among the most accurate approaches for evaluating reaction rate constants in polyatomic systems. To accelerate quantum instanton calculations of kinetic isotope effects, we combine higher-order Boltzmann operator factorization with virial estimators, allowing us to speed up both the convergence to the quantum limit and statistical convergence of the calculation. We estimate the overall resulting acceleration using H+H2/D+D2 as a benchmark system, and then apply the accelerated method to several kinetic isotope effects associated with the H+CH4=H2+CH3 exchange. Last but not least, we explored ways to improve on the quantum instanton approximation for reaction rate constants. To that end, we review quantum instanton and Hansen-Andersen approximations, and propose a combined method, which, as the Hansen-Andersen approximation, has the correct high-temperature behavior, and at the same time, as the quantum instanton approximation, has more flexibility by allowing the dividing surface for the reaction to split into two surfaces at low temperatures. The properties of the combined method are tested on symmetric and asymmetric Eckart barrier.

EPFL2018

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Can accelerometry accurately predict the energy cost of uphill/downhill walking?

Kamiar Aminian, Yves Schutz

To evaluate whether an activity monitor based on body acceleration measurement can accurately assess the energy cost of the human locomotion, 12 subjects walked a combination of three different speeds (preferred speed+/-1 km/h) and seven slopes (-15 to + 15% by steps of 5%) on a treadmill. Body accelerations were recorded using a triaxial accelerometer attached to the low back. The mean of the integral of the vector magnitude (norm) of the accelerations (mIAN) was calculated. (V) over dot O-2 was measured using continuous indirect calorimetry. When the results were separately analysed for each incline, mIAN was correlated to V. O2 (average r = 0.87, p < 0.001, n = 36). (V) over dot O-2 was not significantly correlated to mIAN when data were globally analysed (n = 252). Large relative errors occurred when predicted V. O2 (estimated from data of level walking) was compared with measured (V) over dot O-2 for different inclines (- 53% at +15% incline, to + 55% at - 15% incline). It is concluded that without an external measurement of the slope, the standard method of analysis of body accelerations cannot accurately predict the energy cost of uphill or downhill walking.

2001

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EPFL2018