Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Dropping the Ball: The effect of anisotropic granular materials on
  ejecta and impact crater shape   In this fluid dynamics video, we present an experimental investigation of the
shape of impact craters in granular materials. Complex crater shapes, including
polygons, have been observed in many terrestrial planets as well as moons and
asteroids. We release spherical projectiles from different heights above a
granular bed (sand). The experiments demonstrate two different techniques to
create non-circular impact craters, which we measure by digitizing the final
crater topography. In the first method, we create trenches in the sand to mimic
fault lines or valleys on a planetary target. During impact, ejecta move faster
in the direction of the trenches, creating nearly elliptical craters with the
major axis running parallel to the trench. Larger trenches lead to more oblong
craters. In the second method, a hose beneath the surface of the sand injects
nitrogen gas. The pressure of the gas counters the hydrostatic pressure of the
sand, greatly reducing static friction between grains above the injection
point, without disturbing the surface. The affected sand has lower resistance
to impact, creating a knob in the otherwise circular crater rim.

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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-th
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The categories of this paper are: chem-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: comp-gas
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
13
The categories of this paper are: quant-ph
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The categories of this paper are: astro-ph
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The categories of this paper are: eess
0
The categories of this paper are: hep-ph
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: q-fin
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The categories of this paper are: dg-ga