Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Normal forces in stationary spacetimes   For geodesic motion of a particle in a stationary spacetime the $U_0$
component of particle 4-velocity is constant and is considered to be a
conserved mechanical energy. We show that this concept of a conserved
mechanical energy can be extended to particles that move under the influence of
a ``normal force,'' a force that, in the stationary frame, is orthogonal to the
motion of the particle. We illustrate the potential usefulness of the concept
with a simple example.

17
The categories of this paper are: q-fin
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The categories of this paper are: eess
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
31
The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
11
The categories of this paper are: hep-lat
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ