1. The problem statement, all variables and given/known data Here is a position-time graph for a ball on a spring in SHM. What is the phase of the particle at point a on the graph? 2. Relevant equations phase = ωt + ɸ0 3. The attempt at a solution First I found the initial phase, ɸ0. 0.5A / A = 0.5 = cos(ɸ0). Then ɸ0 = -π/3. It's negative because the particle is moving to the right. Now for point a, I figured this is at 0.75T, where T is the period.This is kind of hard to see so it could be wrong. But then: phase = (2π/T)(0.75T) - (π/3) = (7/6)π or 210 degrees. However, the answer is -120 degrees. I also tried with points b and c but they were wrong too. Can anyone help?