C:\Book\Booktex\C1s3.DVI
65 §1.3 SPECIAL TENSORS Knowing how tensors are defined and recognizing a tensor when it pops up in front of you are two different things. Some quantities, which are tensors, frequently arise in applied problems and you should learn to recognize these special tensors when they occur. In this section some important tensor quantities are defined. We also consider how these special tensors can in turn be used to define other tensors. Metric Tensor Define yi,i=1,...,N as independent coordinates in an N dimensional orthogonal Cartesian coordinate system. The distance squared between two points yi and yi + dyi,i=1,...,N is defined by the expression ds2 = dymdym =(dy1)2 +(dy2)2 + ···+(dyN )2. (1.3.1) Assume that the coordinates yi are related to a set of independent generalized coordinates xi,i=1,...,N by a set of transformation equations yi = yi(x1,x2,...,xN ),i=1,...,N. (1.3.2) To emphasize that each yi depends upon the x coordinates we sometimes use the notation yi = yi(x), for i =1,...,N. The differential of each coordinate can be written as ∂ym dym = dxj ,m=1,...,N, (1.3.3) ∂xj and consequently in the x-generalized coordinates the distance squared, found from the equation (1.3.1), becomes a quadratic form. Substituting equation (1.3.3) into equation (1.3.1) we find ∂ym ∂ym ds2 = dxidxj = g dxidxj (1.3.4) ∂xi ∂xj ij where ∂ym ∂ym g = ,i,j=1,...,N (1.3.5) ij ∂xi ∂xj i are called the metrices of the space defined by the coordinates x ,i=1,...,N.
[Show full text]