Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Storing unsteady energy, like photovoltaically generated electric
  energy, as potential energy   A proposal to store unsteady energy in potential energy via lifting masses
with a rough quantitative overview. Some applications and methods to harvest
the potential energy are also given. A focus is put on photovoltaically
generated energy.

11
The categories of this paper are: hep-lat
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
5
The categories of this paper are: gr-qc
27
The categories of this paper are: comp-gas
21
The categories of this paper are: adap-org
6
The categories of this paper are: astro-ph