Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An Ignored Mechanism for the Longitudinal Recoil Force in Railguns and
  Revitalization of the Riemann Force Law   The electric induction force due to a time-varying current is used to account
for the longitudinal recoil force exerted on the rails of railgun accelerators.
As observed in the experiments, this induction force is longitudinal to the
rails and can be the strongest at the heads of the rails. Besides, for the
force due to a closed circuit, it is shown that the Riemann force law, which is
based on a potential energy depending on a relative speed and is in accord with
Newton's law of action and reaction, can reduce to the Lorentz force law.

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The categories of this paper are: mtrl-th
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The categories of this paper are: supr-con
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: quant-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ex
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The categories of this paper are: solv-int
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The categories of this paper are: ao-sci
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The categories of this paper are: cmp-lg
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The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: econ
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The categories of this paper are: alg-geom