1. The problem statement, all variables and given/known data A mass-spring system with a natural frequency of 3.6 Hz is started in motion with an initial displacement from equilibrium of 6.1 cm and an initial velocity of 0.7 m/s. What is the value of ϕ? (Question aside: Finding the amplitude of the resulting function?) 2. Relevant equations x=Acos(ωt+ϕ) 3. The attempt at a solution Well, I want to say that if we didn't have a initial velocity, then the phase would be 0 rad, because it would simply start at a maximum, and with the cos function, a maximum at time=0 would be 0 phase, so the introduction of the initial condition is having me at a stand still. I would also like to find the amplitude of the resulting motion, and was wondering where to start on this, since the initial displacement cannot be taken to be the amplitude because of the initial velocity in the system.