Fm=kA-->0.8=k*0.4-->k=2N/m
mv^2/2=kA^2/2--->m*0.8^2*π^2=k*0.8^2-->m=2/π^2
ω=(k/m)^(1/2)=π
t=0,x=A/2,v<0--->Φ=π/3
x=0.8cos(πt+π/3)