Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A constructive approach to the special theory of relativity   Simple physical models of a measuring rod and of a clock are used to
demonstrate the contraction of objects and clock retardation in special
relativity. It is argued that the models could help in promoting student
understanding of special relativity and in distinguishing between dynamical and
purely perspectival effects.

13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
0
The categories of this paper are: hep-ph
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
37
The categories of this paper are: supr-con
6
The categories of this paper are: astro-ph
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
23
The categories of this paper are: solv-int
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph