The problem I am having isn't so much related to an exact problem but I am a TA for Dynamics and I was creating a solution set for the current homework where we are dealing with equations of motion using either Normal and Tangential Components or using Cylindrical (2D-Polar) Coordinates. I know students are going to come in and ask how they are supposed to know to use which coordinate system on a test. Maybe you guys can provide me with a better explanation but I probably would tell them that we use the Normal and Tangential components when we know the path that the particle is traveling. Polar seems to be more for solving problems of curvilinear motion when the path is not known. Can you guys please elaborate on this and maybe give me a more educated explanation? I know I am the TA and I should know this class like the back of my hand but I haven't taken it for 4 years. I don't want to look like an idiot. Thanks.