1. The problem statement, all variables and given/known data http://imgur.com/a/FDfAp What is the phase constant? 2. Relevant equations x(t) = A*cos(ωt+Φ) 3. The attempt at a solution If I'm not mistaken at t = 0 the graph starts at half the amplitude or 5. Also the amplitude of this graph is 10, and at t = 0 angular velocity is also 0. 5 = 10*cos(0 + Φ) Φ = arccos(5/10) Φ = π/3 The answer is supposed to be -2π/3, I'm not sure what I did wrong or if I'm even doing the question right.