If vectors A=cos wti + sin wtj and B= cos wt/2 i + sin wt/2 j are functions of time, then the value of t at which they are orthogonal to each other is

Options

(a) t = π/2w
(b) t = π/w
(c) t=0
(d) t = π/4w

Correct Answer:

t = π/w

Explanation:

Two vectors are,
A = cos ωt î + sin ωt ĵ
B = cos (ωt/2) î + sin (ωt/2) ĵ
For two vectors A and B to be orthogonal
A.B = 0
A.B = 0 = cos ωt.cos (ωt/2) + sin ωt.sin (ωt/2)
= cos [ωt – (ωt/2)] = cos (ωt/2)
So, ωt/2 = π/2
.·. t = π/ω.