Block[{m=π (R^2)l ρ,ρ=3000,R=15 (10^(-3)),L=2,l=2 (10^(-2)),Pvac=16000,β=520,R0=15 (10^(-3)),fd=0.06,ρair =1.2},
sol=NDSolve[{x[0]==20,x'[0]==0,m x''[t] ==- π (R^2)(Pvac+β ((R^2)/(R0^2))x'[t]-L fd ((ρair)/2)(((x'[t])^2)/(2R))),WhenEvent[x[t]==0,end=t;"StopIntegration"]},x,{t,0,100}];
Plot[-x'[t]/.sol,{t,0,end},PlotRange->All]
]