2,841 research outputs found
The momentum map for nonholonomic field theories with symmetry
In this note, we introduce a suitable generalization of the momentum map for
nonholonomic field theories and prove a covariant form of the nonholonomic
momentum equation. We show that these covariant objects coincide with their
counterparts in mechanics by making the transition to the Cauchy formalism
The motion of solid bodies in potential flow with circulation: a geometric outlook
The motion of a circular body in 2D potential flow is studied using symplectic reduction. The equations of motion are obtained starting front a kinetic-energy type system on a space of embeddings and reducing by the particle relabelling symmetry group and the special Euclidian group. In the process, we give a geometric interpretation for the Kutta-Joukowski lift force in terms of the curvature of a connection on the original phase space
Multi-Dirac Structures and Hamilton-Pontryagin Principles for Lagrange-Dirac Field Theories
The purpose of this paper is to define the concept of multi-Dirac structures
and to describe their role in the description of classical field theories. We
begin by outlining a variational principle for field theories, referred to as
the Hamilton-Pontryagin principle, and we show that the resulting field
equations are the Euler-Lagrange equations in implicit form. Secondly, we
introduce multi-Dirac structures as a graded analog of standard Dirac
structures, and we show that the graph of a multisymplectic form determines a
multi-Dirac structure. We then discuss the role of multi-Dirac structures in
field theory by showing that the implicit field equations obtained from the
Hamilton-Pontryagin principle can be described intrinsically using multi-Dirac
structures. Furthermore, we show that any multi-Dirac structure naturally gives
rise to a multi-Poisson bracket. We treat the case of field theories with
nonholonomic constraints, showing that the integrability of the constraints is
equivalent to the integrability of the underlying multi-Dirac structure. We
finish with a number of illustrative examples, including time-dependent
mechanics, nonlinear scalar fields and the electromagnetic field.Comment: 50 pages, v2: correction to prop. 6.1, typographical change
Alcohol outlets near schools in a midsize Romanian city : prevalence and characteristics
Objective: alcohol availability is one of the strongest predictors of adolescent alcohol use, and subsequent harm. Alcohol outlets near schools are an important indicator of three types of availability related to adolescent alcohol use; physical (number), economic (price), and legal (compliance with age limits).\ud
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Method: two teams with trained students (16 and 17 years old) visited all 37 schools in a 200,000 inhabitant Romanian city (Pitesti). On the spot all alcohol outlets were visited and data was collected on outlet characteristics and visitors. Also, by conducting mystery shopping purchase attempts by the researchers, compliance on the age limits for alcohol sales was tested.\ud
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Results: a total of 40 outlets were found within a 250 meter distance around 23 schools. Alcohol turns out to be cheap, and commercial alcohol brand signs are dominantly visible. With respect to compliance with the 18-year-old Romanian age limit for alcohol sales, only eight (20%) outlets refused to sell alcohol to under aged decoy customers.\ud
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Conclusion: adolescent alcohol availability is high on the physical, economic and legal aspect. Pitesti is the first city in\ud
Romania where an international alcohol prevention project has started to reduce alcohol related consequences. This project\ud
involves all relevant stakeholders, and the first new legislation on this subject had been implemented
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