So far all the explanations I’ve seen have basically just said, “cause physics”.
In: Physics
You’re going to get a lot of answers that say *conservation of angular momentum* : that, for a given mass (like your arms), the angular momentum of that mass about an axis remains constant, i.e. decreasing the distance between that rotating mass and the axis of rotation increases the speed of rotation. This is true, but doesn’t help you understand *why* things speed up when pulled closer to the axis of rotation. Even when you experience it first hand, it seems counterintuitive.
Inertia, on the other hand, seems intuitive. Ignoring resistance, if a mass is moving it’s going to keep moving in the same direction *at the same speed.*
Focus on the “at the same speed” part of inertia, and then think about the relationship between the circumference of a circle and its radius. An object moving in a straight line with velocity V, tangential to (that is, along the edge of) a circle is moving past it with an *angular velocity* (rotations per of unit time) of V divided by its circumference. If you reduce the size of the circle you decrease its circumference, increasing angular velocity. If you increase the size of the circle you increase its circumference, decreasing angular velocity. The increase (or decrease) in angular velocity is proportional to the decrease (or increase) in the radius of the circle.
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