Question

g 1.32 Two points on a sphere of radius 3 are given as P1(3,0,30) and P2(3,45,45): (a) Find the position vectors of P1 and P2. (b) Find the vector connecting P1 (tail) to P2 (head). (c) Find the position vectors and the vector P1P2 in cylindrical and Cartesian coordinates.

Answer 1

a) P.V of is OP₁ = [ 1.5i + 0j + 2.6k ], P.V of is OP₂ = [ 1.5i + 1.5j + 2.12k ]

b) Vector connecting P₁ to P₂ is [ 0i + 1.5j + 0.48k ]

c) cylindrical coordinates are (1.5, π/2, 0.48)

Explanation:

Given that;

r = 3

P₁ ( 3, 0°, 30° ), P₂ ( 3, 45°, 45° )

a)

P.V of P₁

x = rcos∅sin∅ = 3(cos0°) ( sin30°) = (3 × 1 × 0.5) = 1.5

y = rsin∅sin∅ = 3(sin0°) (sin30°) = (3 × 0 × 0.5) = 0

z = rcos∅ = 3(cos30°) = ( 3 × 0.866) = 2.6

∴ P.V of is OP₁ = [ 1.5i + 0j + 2.6k ]

P.V of P₂

x = rcos∅sin∅ = 3(cos45°) ( sin45°) = (3 × 0.7071 × 0.7071) = 1.5

y = rsin∅sin∅ = 3(sin45°) (sin45°) = (3 × 0.7071 × 0.7071) = 1.5

z = rcos∅ = 3(cos45°) = ( 3 × 0.7071) = 2.12

∴ P.V of is OP₂ = [ 1.5i + 1.5j + 2.12k ]

b)

Vector connecting P₁ to P₂ is given by

OP₂ - OP₁ = [ 1.5i + 1.5j + 2.12k ] - [ 1.5i + 0j + 2.6k ]

= [ 0i + 1.5j + 0.48k ]

c)

P₁P₂ → = [ 0i + 1.5j + 0.48k ] = [ 1.5j + 0.48k ]

so in a cylindrical coordinate, it should be

r = √(o² + 1.5²) = 1.5

∅ = tan⁻¹[y/π] = π/2

z = 0.48

cylindrical coordinates are (1.5, π/2, 0.48)