1. The problem statement, all variables and given/known data a) What is the angular speed ω about the polar axis of a point on Earth's surface at latitude 40° N? (Earth rotates about that axis.) I solved this part b) What is the linear speed v of the point? 2. Relevant equations v = ωr where r = radius of Earth at latitude 40°N 3. The attempt at a solution I found out that r = Rcos(40°) (where R = radius of Earth) but I'm failing to understand why. How am I supposed to know to use cos(40°)? Is it a component of some sort? Where does it come from? If the problem said longitude 40° E or W, would I then use sin(40°)? Also would I have to know to put a negative sign depending on N, S, E, W?