1. The problem statement, all variables and given/known data At 16:30 he writes (x sub 0)(cosine theta). Is x sub 0 the vertical component length of the position vector of the particle? When he labels x sub 0 a little while before, it looks as though x sub 0 is at the radial point along the x axis. How can you use x sub 0 cosine theta at the radial point along the x axis when the position of the particle doesn't coincide with the point at axis? http://ocw.mit.edu/courses/physics/...and-waves-fall-2004/video-lectures/lecture-1/ Also, I know this doesn't adhere to the default question set-up guidelines, but how would I go about properly asking a question like this? I don't do schoolwork. I study what my weak mind can grasp alone at a pace at which I find most comfortable. I really don't know what he meant by this.