text: how do you find the phase angle of a spring-mass system in oscillation? ok. here's the set up: there's a spring-mass system that's oscillating. at time t=0, the mass is at position x=0 (the equilibrium point) and is moving with some positive velocity.  what is the phase angle? the phase angle is the displacement of the wave (delta) from it's normal position corresponding to delta=0.  for example if  the solution is the form acos(wt)+bsin(wt) then it can be rewritten in the form rcos(wt-delta) where r=(a^2+b^2)^.5 and tan(delta)=b/a and w^2=k/m