Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Rigid body dynamics of railway ballast   A method for the discrete particle simulation of of almost rigid, sharply
edged frictional particles, such as railway ballast is proposed. In difference
to Molecular Dynamics algorithms, the method does not require knowledge about
the deformation-force law of the material. Moreover, the method does not suffer
from numerical instability which is encountered in MD simulations of very stiff
particles.

11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
0
The categories of this paper are: hep-ph
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
19
The categories of this paper are: eess
28
The categories of this paper are: alg-geom
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin
5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ