1. The problem statement, all variables and given/known data What is the equation for the displacement of a sin wave (eg something in SHM)? My book says x(t) = Acos(ωt + ɸ0) My prof's notes say y(x,t) = Asin(ωt - kx + ɸ0) 2. Relevant equations 3. The attempt at a solution I'm not confused about why one is sin and the other is cos. I'm confused why one has the extra term -kx. When do you use which? I have a question that asks me to write the displacement for a sine wave travelling to the left with frequency 200 Hz, speed 400 m/s, amplitude 0.010 m, and initial phase 90 degrees. So I'm not sure which of the above to use.