Rearuh

Euler, Lagrange, and Kovalevskaya

In classical mechanics, the precession of a rigid body such as a top under the influence of gravity is not, in general, an integrable problem. There are however three (or four) famous cases that are integrable, the Euler, the Lagrange, and the Kovalevskaya top.^[1][2] 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 in which the fixed point is the center of gravity. The Lagrange top is a symmetric top, in which two moments of inertia are the same and the center of gravity lies on the symmetry axis. The Kovalevskaya top^[3][4] is a special symmetric top with a unique ratio of the moments of inertia which satisfy the relation

I1=I2=2I3,{displaystyle I_{1}=I_{2}=2I_{3},}

That is, two moments of inertia are equal, the third is half as large, and the center of gravity is located in the plane perpendicular to the symmetry axis (parallel to the plane of the two equal points). The nonholonomic Goryachev–Chaplygin top (introduced by D. Goryachev in 1900^[5] and integrated by Sergey Chaplygin in 1948^[6][7]) is also integrable (I1=I2=4I3{displaystyle I_{1}=I_{2}=4I_{3}}). Its center of gravity lies in the equatorial plane.^[8]

Hamiltonian formulation of classical tops

A classical top^[9] is defined by three principal axes, defined by the three orthogonal vectors e^1{displaystyle {hat {mathbf {e} }}^{1}}, e^2{displaystyle {hat {mathbf {e} }}^{2}} and e^3{displaystyle {hat {mathbf {e} }}^{3}} with corresponding moments of inertia I1{displaystyle I_{1}}, I2{displaystyle I_{2}} and I3{displaystyle I_{3}}. In a Hamiltonian formulation of classical tops, the conjugate dynamical variables are the components of the angular momentum vector L→{displaystyle {vec {L}}} along the principal axes

(ℓ1,ℓ2,ℓ3)=(L→⋅e^1,L→⋅e^2,L→⋅e^3){displaystyle (ell _{1},ell _{2},ell _{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,

(n1,n2,n3)=(z^⋅e^1,z^⋅e^2,z^⋅e^3){displaystyle (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

{ℓa,ℓb}=εabcℓc, {ℓa,nb}=εabcnc, {na,nb}=0{displaystyle {ell _{a},ell _{b}}=varepsilon _{abc}ell _{c}, {ell _{a},n_{b}}=varepsilon _{abc}n_{c}, {n_{a},n_{b}}=0}

If the position of the center of mass is given by R→cm=(ae^1+be^2+ce^3){displaystyle {vec {R}}_{cm}=(amathbf {hat {e}} ^{1}+bmathbf {hat {e}} ^{2}+cmathbf {hat {e}} ^{3})}, then the Hamiltonian of a top is given by

H=(ℓ1)22I1+(ℓ2)22I2+(ℓ3)22I3+mg(an1+bn2+cn3),{displaystyle H={frac {(ell _{1})^{2}}{2I_{1}}}+{frac {(ell _{2})^{2}}{2I_{2}}}+{frac {(ell _{3})^{2}}{2I_{3}}}+mg(an_{1}+bn_{2}+cn_{3}),}

The equations of motion are then determined by

ℓ˙a={H,ℓa},n˙a={H,na}{displaystyle {dot {ell }}_{a}={H,ell _{a}},{dot {n}}_{a}={H,n_{a}}}

Euler top

The Euler top is an untorqued top, with Hamiltonian

HE=(ℓ1)22I1+(ℓ2)22I2+(ℓ3)22I3,{displaystyle H_{E}={frac {(ell _{1})^{2}}{2I_{1}}}+{frac {(ell _{2})^{2}}{2I_{2}}}+{frac {(ell _{3})^{2}}{2I_{3}}},}

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

(L1,L2,L3)=ℓ1e^1+ℓ2e^2+ℓ3e^3.{displaystyle (L_{1},L_{2},L_{3})=ell _{1}mathbf {hat {e}} ^{1}+ell _{2}mathbf {hat {e}} ^{2}+ell _{3}mathbf {hat {e}} ^{3}.}

Lagrange top

"Physically, the Lagrange top is a symmetric rigid body spinning about its figure axis whose base point is fixed. A constant vertical gravitational force acts on the center of mass of the top, which lies on its axis."^[10]

The Lagrange top (so named after Joseph-Louis Lagrange) is a symmetric top with the center of mass along the symmetry axis at location, R→cm=he^3{displaystyle {vec {R}}_{cm}=hmathbf {hat {e}} ^{3}}, with Hamiltonian

HL=(ℓ1)2+(ℓ2)22I+(ℓ3)22I3+mghn3.{displaystyle H_{L}={frac {(ell _{1})^{2}+(ell _{2})^{2}}{2I}}+{frac {(ell _{3})^{2}}{2I_{3}}}+mghn_{3}.}

The four constants of motion are the energy HL{displaystyle H_{L}}, the angular momentum component along the symmetry axis, ℓ3{displaystyle ell _{3}}, the angular momentum in the z-direction

Lz=ℓ1n1+ℓ2n2+ℓ3n3,{displaystyle L_{z}=ell _{1}n_{1}+ell _{2}n_{2}+ell _{3}n_{3},}

and the magnitude of the n-vector

n2=n12+n22+n32{displaystyle n^{2}=n_{1}^{2}+n_{2}^{2}+n_{3}^{2}}

Kovalevskaya top

The Kovalevskaya top^[3][4] is a symmetric top in which I1=I2=2I3=I{displaystyle I_{1}=I_{2}=2I_{3}=I} and the center of mass lies in the plane perpendicular to the symmetry axis R→cm=he^1{displaystyle {vec {R}}_{cm}=hmathbf {hat {e}} ^{1}}. It was discovered by Sofia Kovalevskaya in 1888 and presented in her paper "Sur le problème de la rotation d'un corps solide autour d'un point fixe", which won the Prix Bordin from the French Academy of Sciences in 1888. The Hamiltonian is

HK=(ℓ1)2+(ℓ2)2+2(ℓ3)22I+mghn1.{displaystyle H_{K}={frac {(ell _{1})^{2}+(ell _{2})^{2}+2(ell _{3})^{2}}{2I}}+mghn_{1}.}

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

K=ξ+ξ−{displaystyle K=xi _{+}xi _{-}}

where the variables ξ±{displaystyle xi _{pm }} are defined by

ξ±=(ℓ1±iℓ2)2−2mghI(n1±in2),{displaystyle xi _{pm }=(ell _{1}pm iell _{2})^{2}-2mghI(n_{1}pm in_{2}),}

the angular momentum component in the z-direction,

Lz=ℓ1n1+ℓ2n2+ℓ3n3,{displaystyle L_{z}=ell _{1}n_{1}+ell _{2}n_{2}+ell _{3}n_{3},}

and the magnitude of the n-vector

n2=n12+n22+n32.{displaystyle n^{2}=n_{1}^{2}+n_{2}^{2}+n_{3}^{2}.}

See also

• Cardan suspension

References

External links

• Kovalevskaya Top – from Eric Weisstein's World of Physics