6,957 research outputs found
The force, power and energy of the 100 meter sprint
At the 2008 Summer Olympics in Beijing, Usain Bolt broke the world record for
the 100 m sprint. Just one year later, at the 2009 World Championships in
Athletics in Berlin he broke it again. A few months after Beijing, Eriksen et
al. studied Bolt's performance and predicted that Bolt could have run about
one-tenth of a second faster, which was confirmed in Berlin. In this paper we
extend the analysis of Eriksen et al. to model Bolt's velocity time-dependence
for the Beijing 2008 and Berlin 2009 records. We deduce the maximum force, the
maximum power, and the total mechanical energy produced by Bolt in both races.
Surprisingly, we conclude that all of these values were smaller in 2009 than in
2008
A unified model for the long and high jump
A simple model based on the maximum energy that an athlete can produce in a
small time interval is used to describe the high and long jump. Conservation of
angular momentum is used to explain why an athlete should run horizontally to
perform a vertical jump. Our results agree with world records.Comment: Accepted for publication in Am. J. Phy
Understanding the tsunami with a simple model
In this paper, we use the approximation of shallow water waves (Margaritondo
G 2005 Eur. J. Phys. 26 401) to understand the behaviour of a tsunami in a
variable depth. We deduce the shallow water wave equation and the continuity
equation that must be satisfied when a wave encounters a discontinuity in the
sea depth. A short explanation about how the tsunami hit the west coast of
India is given based on the refraction phenomenon. Our procedure also includes
a simple numerical calculation suitable for undergraduate students in physics
and engineering
Characterization of the regulatory region of bovine mucin 2 (MUC2) gene.
Na publicação: E. O. Melo
Characterization of thermoplastic starch/poly(lactic acid) blends obtained by extrusion and thermopressing
Blends of poly(lactic acid) (PLA)/thermoplastic starch (TPS) were prepared by extrusion and thermopressing, with proportions of 0.1, 0.2 and 0.3 g of PLA per g TPS. Thermoplastic starch was obtained from the extrusion of 0.30 g or 0.25 g of glycerol per g of starch. The mechanical properties and water vapor permeability were tested and water sorption isotherms were obtained. The morphological characteristics of the samples were observed through scanning electron microscopy (SEM) and the thermal properties of the pure polymers and the blends by differential scanning calorimetry (DSC). The SEM and DSC results indicated the immiscibility of the TPS/PLA blends and the presence of two phases. For blends with 20% and 30% of PLA increases in the tensile strength and modulus were observed. The lowest water vapor permeability was observed for the blend with 30% of PLA, which is related to the lowest diffusion coefficient value observed for this sample
Componentes produtivos de cultivares de feijão comum em cultivo safrinha.
O presente trabalho teve por objetivo avaliar os componentes produtivos de cultivares de feijão comum de diversos grupos comerciais na época de safrinha
Feijão comum: características morfo-agronômicas de cultivares.
O presente trabalho objetivou avaliar as características morfoagronômicas de cultivares de feijão comum de diversos grupos comerciais na época de safrinha
Stabilization of the number of Bose-Einstein condensed atoms in evaporative cooling via three-body recombination loss
The dynamics of evaporative cooling of magnetically trapped Rb atoms
is studied on the basis of the quantum kinetic theory of a Bose gas. We carried
out the quantitative calculations of the time evolution of conventional
evaporative cooling where the frequency of the radio-frequency magnetic field
is swept exponentially. This "exponential-sweep cooling" is known to become
inefficient at the final stage of the cooling process due to a serious
three-body recombination loss. We precisely examine how the growth of a
Bose-Einstein condensate depends on the experimental parameters of evaporative
cooling, such as the initial number of trapped atoms, the initial temperature,
and the bias field of a magnetic trap. It is shown that three-body
recombination drastically depletes the trapped Rb atoms as the system
approaches the quantum degenerate region and the number of condensed atoms
finally becomes insensitive to these experimental parameters. This result
indicates that the final number of condensed atoms is well stabilized by a
large nonlinear three-body loss against the fluctuations of experimental
conditions in evaporative cooling.Comment: 7 pages, REVTeX4, 8 eps figures, Phys. Rev A in pres
- …