Let us focus on the velocity only. Discretizing and using the simplest possible approximation for the derivative, we have Euler's forward method for the updated velocity at a time step i+1 given by
vi+1=vi+Δtdvdt|v=vi=vi+Δt(Fi−ηvi−xi).Defining a function
hi(xi,vi,Fi)=vi+Δt(Fi−ηvi−xi),we have
vi+1=hi(xi,vi,Fi).