Simple Harmonic Motion - whats the displacement of the mass

[astrozilla]

Homework Statement

A mass on a spring obeys SHM.Given that the mass is initially at the equilibrium position,has an initial velocity of 2m/s,and a nastural frequency of 2 rad/s,Find A and φ
After 13.2 sec what's the displacement of the mass

Homework Equations

ψ=Αcos(ωt+φ)

The Attempt at a Solution

Is the initial velocity ,the maximum velocity ? Maybe yes. Is the initial phase (φ) zero ? I think so.

 

[Plutoniummatt]

the initial velocity IS the maximum velocity, at equilibrium, KE = max, PE = zero.

if you use your notation for the motion of the mass, ie the cosine term, then no, the phase is not zero, it has to be pi/2, because at T=0, psi must be zero because the mass is at equilibrium.

 

[astrozilla]

Thanks
Problem solved :)