APPENDICES
APPENDIX A
THE METHOD OF CURVILINEAR COORDINATES
In Appendix A the kinematics of point in curvilinear coordinates is consid- ered. The formulas obtained are extended to the motion of any of mechanical systems. The theory, given in the Appendix, is widely used in studying the base material of the monograph.
§ 1. The curvilinear coordinates of point. Reciprocal bases
Suppose, the position of the point M in three-dimensional space is defined by the radius-vector r = r(q1,q2,q3), i. e. the Cartesian coordinates of point 1 2 3 x1,x2,x3 are uniquely represented via the quantities q ,q ,q :
1 2 3 xk = Fk(q ,q ,q ) ,k=1, 2, 3 . (A.1)
If ∂x1 ∂x1 ∂x1 ∂q1 ∂q2 ∂q3 D(x ,x ,x ) ∂x ∂x ∂x 1 2 3 = 2 2 2 =0 , 1 2 3 1 2 3 D(q ,q ,q ) ∂q ∂q ∂q ∂x ∂x ∂x 3 3 3 ∂q1 ∂q2 ∂q3 then system of equations (A.1) is solvable for q1,q2,q3:
σ σ q = f (x1,x2,x3) ,σ=1, 2, 3 , (A.2) and the quantities q1,q2,q3 are called the curvilinear coordinates of point in space. From relations (A.2) it follows directly that, equating any curvilinear co- σ ordinate q to the constant quantity Cσ, we obtain the equation of coordinate surface σ f (x1,x2,x3)=Cσ ,σ=1, 2, 3 . The crossing of two coordinate surfaces gives a coordinate line, along which one coordinate is varied only. For example, the crossing of the coordinate 1 1 2 2 surfaces q = f (x1,x2,x3)=C1 and q = f (x1,x2,x3)=C2 gives a coordinate line, which the coordinate q3 varies along (Fig. A. 1).
213 214 Appendix A
Fig. A. 1
The crossing of the coordinate lines q1,q2,q3 is the point M. If through this point we construct the tangents to the coordinate lines in ascending order of the quantities q1,q2,q3,thenweobtainthe axes of curvilinear coordinates, which can make up as orthogonal (for example, the axes of spherical or cylin- drical coordinates) as nonorthogonal systems. For the motion to be given in curvilinear coordinates it is necessary that the quantities q1,q2,q3 are given as time functions: qσ = qσ(t) ,σ= 1, 3 . (A.3) These functions are called equations of motion of point. Taking into account that the radius-vector r = r(q1,q2,q3) of the point M is a differentiable function, we obtain