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In classical mechanics, the precession of a top under the influence of gravity is not in general, an integrable problem. There are however three famous cases that are integrable, the Euler, the Lagrange and the Kovalevskaya top. In addition to the energy, each of these tops involves three additional constants of motion that give rise to the integrability.

The Euler top describes a free top without any particular symmetry, moving in the absence of any external torque. The Lagrange top is a symmetric top, in which the center of gravity lies on the symmetry axis. The Kovalevskaya top is special symmetric top with a unique ratio of the moments of inertia satisfy the relation

$I_{1}=I_{2}=2I_{3}$,

and in which the center of gravity is located in the plane perpendicular to the symmetry axis.

Hamiltonian Formulation of Classical tops

A classical top is defined by three principal axes, defined by the three orthogonal vectors ${\hat {\mathbf {e} }}^{1}$, ${\hat {\mathbf {e} }}^{2}$ and ${\hat {\mathbf {e} }}^{3}$ with corresponding moments of inertia $I_{1}$, $I_{2}$ and $I_{3}$. In a Hamiltonian formulation of classical tops, the conjugate dynamical variables are the components of the angular momentum vector ${\vec {L}}$ along the principal axes

$(l_{1},l_{2},l_{3})=({\vec {L}}\cdot {\hat {\mathbf {e} }}^{1},{\vec {L}}\cdot {\hat {\mathbf {e} }}^{2},{\vec {L}}\cdot {\hat {\mathbf {e} }}^{3})$

and the z-components of the three principal axes,

$(n_{1},n_{2},n_{3})=(\mathbf {\hat {z}} \cdot {\hat {\mathbf {e} }}^{1},\mathbf {\hat {z}} \cdot {\hat {\mathbf {e} }}^{2},\mathbf {\hat {z}} \cdot {\hat {\mathbf {e} }}^{3})$

The Poisson algebra of these variables is given by

$\{l_{a},l_{b}\}=\epsilon _{abc}l_{c},\ \{l_{a},n_{b}\}=\epsilon _{abc}n_{c},\ \{n_{a},n_{b}\}=0$

If the position of the center of mass is given by ${\vec {R}}_{cm}=(a\mathbf {\hat {e}} ^{1}+b\mathbf {\hat {e}} ^{2}+c\mathbf {\hat {e}} ^{3})$, then the Hamiltonian of a top is given by

$H={\frac {(l_{1})^{2}}{2I_{1}}}+{\frac {(l_{2})^{2}}{2I_{2}}}+{\frac {(l_{3})^{2}}{2I_{3}}}+mg(an_{1}+bn_{2}+cn_{3}),$

The equations of motion are then determined by

${\dot {l}}_{a}=\{H,l_{a}\},{\dot {n}}_{a}=\{H,n_{a}\}$

Euler Top

The Euler top is an untorqued top, with Hamiltonian

$H_{E}={\frac {(l_{1})^{2}}{2I_{1}}}+{\frac {(l_{2})^{2}}{2I_{2}}}+{\frac {(l_{3})^{2}}{2I_{3}}},$

The four constants of motion are the energy $H_{E}$ and the three components of angular momentum in the lab frame,

$(L_{1},L_{2},L_{3})=l_{1}\mathbf {\hat {e}} ^{1}+l_{2}\mathbf {\hat {e}} ^{2}+l_{3}\mathbf {\hat {e}} ^{3}.$

Lagrange Top

The Lagrange top is a symmetric top with the center of mass along the symmetry axis at location, ${\vec {R}}_{cm}=h\mathbf {\hat {e}} ^{3}$, with Hamiltonian

$H_{L}={\frac {(l_{1})^{2}+(l_{2})^{2}}{2I}}+{\frac {(l_{3})^{2}}{2I_{3}}}+mghn_{3}.$

The four constants of motion are the energy $H_{L}$, the angular momentum component along the symmetry axis, $l_{3}$, the angular momentum in the z-direction

$L_{z}=l_{1}n_{1}+l_{2}n_{2}+l_{3}n_{3},$

and the magnitude of the n-vector

$n^{2}=n_{1}^{2}+n_{2}^{2}+n_{3}^{2}$

Kovalevskaya Top

The Kovalevskaya top is a symmetric top in which $I_{1}=I_{2}=2I_{3}=I$ and the center of mass lies in the plane perpendicular to the symmetry axis ${\vec {R}}_{cm}=h\mathbf {\hat {e}} ^{1}$. The Hamiltonian is

$H_{K}={\frac {(l_{1})^{2}+(l_{2})^{2}+2(l_{3})^{2}}{2I}}+mghn_{1}.$

The four constants of motion are the energy $H_{K}$, the Kovalevskaya invariant

$K=\xi _{+}\xi _{-}$

where the variables $\xi _{\pm }$ are defined by

$\xi _{\pm }=(l_{1}\pm il_{2})^{2}-2mghI(n_{1}\pm in_{2}),$

the angular momentum component in the z-direction,

$L_{z}=l_{1}n_{1}+l_{2}n_{2}+l_{3}n_{3},$

and the magnitude of the n-vector

$n^{2}=n_{1}^{2}+n_{2}^{2}+n_{3}^{2}.$

References

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