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Basic Concepts of Vector Algebra

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Estimated time: 2 minutes
  • Position Vector
  • Direction Cosines and Direction Ratios of a Vector

Notes

Let ‘l’ be any straight line in plane or three dimensional space. This line can be given two directions by means of arrowheads. A line with one of these directions prescribed is called a directed line in following  first and second fig .

Position Vector:
The three dimensional right handed rectangular coordinate system in  following first fig.

Consider a point P in space, having coordinates (x, y, z) with respect to the origin O(0, 0, 0). Then, the vector  having O and P as its initial and terminal points, respectively, is called the position vector of the point P with respect to O. 
Using distance formula `|vec (OP)| = sqrt(x^2 + y^2 + z^2)`
In practice, the position vectors of points A, B, C, etc., with respect to the origin O are denoted by `vec a , vec b , vec c` etc., respectively in above second fig.

Direction Cosines:
The position vector `vec (OP) (or vec r)` of a point P(x, y, z) as in  following Fig.

The angles α, β, γ made by the vector `vec r` with the positive directions of x, y and z-axes respectively, are called its direction angles. 
The cosine values of these angles, i.e., cosα, cosβ and cosγ are called direction cosines of the vector `vec r` , and usually denoted by l, m and n, respectively.

one may note that the triangle OAP is right angled, and in it, we have `cos alpha = x/r`( r stands for |`vec r`|).  Similarly, from the right angled triangles OBP and OCP, we may write  `cos beta= y/r` and `cos gamma = z/r`.  Thus, the coordinates of the point P may also be expressed as (lr, mr,nr).  The numbers lr, mr and nr, proportional to the direction cosines are called as direction ratios of vector `vec r` , and denoted as a, b and c, respectively.

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