I am trying to work the following problem; A rigid body is rotating about a fixed axis with a constant angular velocity ω. Take ω to lie entirely on th z-axis. Express r in cylindrical coordinates, and calculate; a) v=ω × r b)∇ × v The answer to (a) is v=ψωρ and (b) is ∇ × v = 2ω Firstly, I am not sure if we need r=[ρcosψ, psinψ, z] or simply [p, ψ, z]. Secondly, are they saying that ω is simply [0, 0, ω]? if so then I don't get the result so I am doing something wrong. Any help? I am sure that tackling this problem will deepen my understanding of curvilinear coordinates.