Gaurav Sharma
Last Activity: 15 Years ago
Properties of scalar product
Property 1 :
The scalar product of two vectors is commutative
av.bv = bv.av
Property 2 : Scalar Product of Collinear Vectors :
(i) When the vectors av and bv are collinear and are in the same direction, then θ = 0
av.bv = |av| |bv| = ab
(i) When the vectors av and bv are collinear and are in the opposite direction, then θ = π
av.bv = |av| |bv|(-1) = -ab
Property 3 : Sign of Dot Product
The dot product av.bv may be positive or negative or zero.
(i) If the angle between the two vectors is acute (i.e., 0 < θ < 90°) then
cos θ is positive. In this case dot product is positive.
(ii) If the angle between the two vectors is obtuse (i.e., 90 < θ < 180) then
cos θ is negative. In this case dot product is negative.
(iii) If the angle between the two vectors is 90° (i.e., θ = 90°) then
cos θ = cos 90° = 0. In this case dot product is zero.
scalar product in terms of components
If a = a1i+a2j+a3k and
b= b1i+b2j+b3k
then a.b = a1b1+a2b2+a3b3
Angle between two vectors
If θ is the angle between two vectors,
cos θ = a.b/|a||b|
=> θ = cos-1 (a.b/|a||b|)
In component form
If a = a1i+a2j+a3k and
b= b1i+b2j+b3k
θ = cos-1[(a1b1+a2b2+a3b3)/(SQRT(a1²+a2²+a3²)*SQRT(b1²+b2²+b3²))
Components of a vector b along and perpendicular to vector a
Component of vector b along vector a == (a.b/|a|²)aComponent of vector b perpendicular to vector a = b- (a.b/|a|²)a