By Forsyth A.R.
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Extra info for Theory of differential equations. Ordinary nonlinear equations
Example text
The a-pivot can rotate freely in the (x, y)-plane while the b-pivot can rotate freely in a vertical plane x1 = a sin ϕ − b cos θ sin ϕ, x2 = a sin ϕ + b cos θ sin ϕ, y1 = a cos ϕ − b cos θ cos ϕ, y2 = a cos ϕ + b cos θ cos ϕ, z 1 = b sin θ ; z 2 = −b sin θ. 190). 191) we obtain the action of the Thomson–Tait pendulum S= dtm b2 θ˙ 2 + (a 2 + b2 cos2 θ )ϕ˙ 2 . 193) This implies the equations of motion (a 2 + b2 cos2 θ )ϕ˙ ˙ = 0, θ¨ + ϕ˙ 2 cos θ sin θ = 0. 194) Note that it would not be an easy task to find these equations directly, without using variational analysis.
It obeys the properties q a (τ, 0) = q a (τ ), q a (τ, 1) = q a (τ ) + δq a (τ ). With the functional S we associate the usual function7 of the variable s q(τ2) q(τ) + δq(τ) q(τ, s) q(τ) q(τ1) Fig. 7 A one-parameter family q(τ, s) connecting the curves q(τ ) and q(τ ) + δq(τ ) 7 Of course S(s) depends also on the choice of q(τ ) and δq(τ ). 26 1 Sketch of Lagrangian Formalism S(s) ≡ S[q a + sδq a ], s [−1, 1]. 99) Then the variation δS of the functional S “at the point q a (τ )” is defined by the formula δS[q] ≡ d S(s) ds = s=0 d S[q + sδq] ds .
177) 1 this reduces to the well-known g x = 0. 178) Let us discuss the recipe from the geometric point of view. The constraint x 2+y 2 =l 2 represents the equation of a circle in configuration space. √ The variable x can be taken as a system of coordinates on the line, then y = − l 2 − x 2 is a parametric equation of the line (in the vicinity of the point we are interested in). This solves the constraint. So, geometrically the recipe consists of restricting the unconstrained Lagrangian function on the line.