To find velocity:
a=dv/dt=-0.4v^3
integral of dv/-0.4v^3
1/-0.4(1/-2)1/v^2 with integral limit things being 0 and 60= t-0
1/0.8(1/v^2-1/60^2)= t
v= ((1/60^2+0.8t)^-1/2)m/s
when t=4s,
v=0.559m/s downwards
To find position:
v=sd/dt= ((1/60^2+0.8t)^-1/2)
integral of (1/60^2+0.8t)^-1/2dt...