Could someone please explain why PQ in the diagram below is rΔθ? Isn't rΔθ arc length? The best reason I can think of is that it's only an approximation for when the angle is very small, so PQ≈arclength=rΔθ. Not 100% sure though. http://imageshack.us/scaled/landing/199/feynmanangle.jpg [Broken] The diagram is from the first volume of the Feynman lectures in 18-3, in the section where he talks about rotation of rigid bodies.