I'm reading Advanced Calculus by Wilfred Kaplan 1952. He is demonstrating how to find the decomposition of the acceleration vector into its normal and tangential components. I'm following along until he replaces the magnitude of the derivative of the angle with respect to the distance traveled by the particle along the curve with 1/p where p is the radius of curvature of the path. Later in the book, he mentions Frenet formulas. Can anyone explain what these Frenet formulas are about and what he means by radius of curvature of the path? Or did I not supply enough information? Thanks.