12,396 research outputs found
Effect of Bilayer Thickness on Membrane Bending Rigidity
The bending rigidity of bilayer vesicles self-assembled from
amphiphilic diblock copolymers has been measured using single and
dual-micropipet techniques. These copolymers are nearly a factor of 5 greater
in hydrophobic membrane thickness than their lipid counterparts, and an
order of magnitude larger in molecular weight . The macromolecular
structure of these amphiphiles lends insight into and extends relationships for
traditional surfactant behavior. We find the scaling of with thickness to
be nearly quadratic, in agreement with existing theories for bilayer membranes.
The results here are key to understanding and designing soft interfaces such as
biomembrane mimetics
The role of sign in students' modeling of scalar equations
We describe students revising the mathematical form of physics equations to
match the physical situation they are describing, even though their revision
violates physical laws. In an unfamiliar air resistance problem, a majority of
students in a sophomore level mechanics class at some point wrote Newton's
Second Law as F = -ma; they were using this form to ensure that the sign of the
force pointed in a direction consistent with the chosen coordinate system while
assuming that some variables have only positive value. We use one student's
detailed explanation to suggest that students' issues with variables are
context-dependent, and that much of their reasoning is useful for productive
instruction.Comment: 5 pages, 1 figure, to be published in The Physics Teache
A study of commuter airplane design optimization
Problems of commuter airplane configuration design were studied to affect a minimization of direct operating costs. Factors considered were the minimization of fuselage drag, methods of wing design, and the estimated drag of an airplane submerged in a propellor slipstream; all design criteria were studied under a set of fixed performance, mission, and stability constraints. Configuration design data were assembled for application by a computerized design methodology program similar to the NASA-Ames General Aviation Synthesis Program
Beyond deficit-based models of learners' cognition: Interpreting engineering students' difficulties with sense-making in terms of fine-grained epistemological and conceptual dynamics
Researchers have argued against deficit-based explanations of students'
troubles with mathematical sense-making, pointing instead to factors such as
epistemology: students' beliefs about knowledge and learning can hinder them
from activating and integrating productive knowledge they have. In this case
study of an engineering major solving problems (about content from his
introductory physics course) during a clinical interview, we show that "Jim"
has all the mathematical and conceptual knowledge he would need to solve a
hydrostatic pressure problem that we posed to him. But he reaches and sticks
with an incorrect answer that violates common sense. We argue that his lack of
mathematical sense-making-specifically, translating and reconciling between
mathematical and everyday/common-sense reasoning-stems in part from his
epistemological views, i.e., his views about the nature of knowledge and
learning. He regards mathematical equations as much more trustworthy than
everyday reasoning, and he does not view mathematical equations as expressing
meaning that tractably connects to common sense. For these reasons, he does not
view reconciling between common sense and mathematical formalism as either
necessary or plausible to accomplish. We, however, avoid a potential "deficit
trap"-substituting an epistemological deficit for a concepts/skills deficit-by
incorporating multiple, context-dependent epistemological stances into Jim's
cognitive dynamics. We argue that Jim's epistemological stance contains
productive seeds that instructors could build upon to support Jim's
mathematical sense-making: He does see common-sense as connected to formalism
(though not always tractably so) and in some circumstances this connection is
both salient and valued.Comment: Submitted to the Journal of Engineering Educatio
Ascent control studies of the 049 and ATP parallel burn solid rocket motor shuttle configurations
The control authority approach is discussed as a major problem of the parallel burn soil shuttle configuration due to the many resulting system impacts regardless of the approach. The major trade studies and their results, which led to the recommendation of an SRB TVC control authority approach are presented
Moller operators and Lippmann-Schwinger equations for step-like potentials
The Moller operators and the asociated Lippman-Schwinger equations obtained
from different partitionings of the Hamiltonian for a step-like potential
barrier are worked out, compared and related.Comment: 15 pages, 1 inlined figure, iopart.cl
Symbolic Manipulators Affect Mathematical Mindsets
Symbolic calculators like Mathematica are becoming more commonplace among
upper level physics students. The presence of such a powerful calculator can
couple strongly to the type of mathematical reasoning students employ. It does
not merely offer a convenient way to perform the computations students would
have otherwise wanted to do by hand. This paper presents examples from the work
of upper level physics majors where Mathematica plays an active role in
focusing and sustaining their thought around calculation. These students still
engage in powerful mathematical reasoning while they calculate but struggle
because of the narrowed breadth of their thinking. Their reasoning is drawn
into local attractors where they look to calculation schemes to resolve
questions instead of, for example, mapping the mathematics to the physical
system at hand. We model the influence of Mathematica as an integral part of
the constant feedback that occurs in how students frame, and hence focus, their
work
Chandra X-ray Observations of Galaxies in an Off-Center Region of the Coma Cluster
We have performed a pilot Chandra survey of an off-center region of the Coma
cluster to explore the X-ray properties and Luminosity Function of normal
galaxies. We present results on 13 Chandra-detected galaxies with optical
photometric matches, including four spectroscopically-confirmed Coma-member
galaxies. All seven spectroscopically confirmed giant Coma galaxies in this
field have detections or limits consistent with low X-ray to optical flux
ratios (fX/fR < 10^-3). We do not have sufficient numbers of X-ray detected
galaxies to directly measure the galaxy X-ray Luminosity Function (XLF).
However, since we have a well-measured optical LF, we take this low X-ray to
optical flux ratio for the 7 spectroscopically confirmed galaxies to translate
the optical LF to an XLF. We find good agreement with Finoguenov et al. (2004),
indicating that the X-ray emission per unit optical flux per galaxy is
suppressed in clusters of galaxies, but extends this work to a specific
off-center environment in the Coma cluster. Finally, we report the discovery of
a region of diffuse X-ray flux which might correspond to a small group
interacting with the Coma Intra-Cluster Medium (ICM).Comment: Accepted for publication in the Astrophysical Journa
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