Hello, I was working on a classical mechanics problem, and I suddenly came up with this question: Let's think about a point in a 2-D plane. This point is represented by (r, θ), where r is the distance between the origin and the point and θ is the angle measured from the x-axis. Now, what is the significance of the angle θ if r=0? Personally, I do not think θ has any significance because θ do not affect any property of the point since it is located exactly at the origin. To me, it makes no sense at all to specify a point at the origin with an angle. Is there any case where θ has any physical(or mathematical) significance although r=0?