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FirstIntegrals

FirstIntegrals[f, x[t], t]
returns a list of first integrals corresponding to the coordinate x[t] and independent variable t of the integrand f.
FirstIntegrals[f, {x[t], y[t], ...}, t]
returns a list of first integrals corresponding to the coordinates x, y, ... and independent variable t.
  • A first integral is a conserved quantity associated with a coordinate or the independent variable.
  • A first integral associated with a coordinate x[t], y[t], ... is returned if f is independent of that coordinate, although f may contain derivatives of the coordinate. Such coordinates are typically called cyclic or ignorable coordinates.
  • A first integral associated with the independent variable t is returned if f is independent of t and does not contain any second or higher derivatives of the coordinates.
  • In mechanics, a first integral corresponding to a coordinate is typically associated with conservation of momentum, and a first integral corresponding to the independent variable is typically associated with conservation of energy.
  • FirstIntegrals returns a list of rules of the form FirstIntegral[u]->c, where u may be either the coordinates x, y, ... or the independent variable t, and c is the conserved quantity.
Needs["VariationalMethods`"]
The Lagrangian of a particle in 2-dimensions with a central potential:
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The coordinates with conserved first integrals are the angle and the time t, corresponding to conservation of angular momentum and energy:
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Needs["VariationalMethods`"]
The area of a surface of revolution obtained by revolving the curve y[x] about the x-axis has the integrand:
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Here f has no explicit dependence on x:
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