Mathematical Particle tracing - resintence force

Hi!

I'm trying to compute some trajectories using Comsol Math. Particle Tracing in the Newtonian formulation for some force which is of the form:
F=F1+F2
where F1 is proportional with the dielectrophoresis force
and F2 is a drag force proportional with the velocity.

my question is: haw do I introduce a time derivative in Math PT? to simulate a force proportional with velocity or haw do I introduce a velocity term in the force?

I've tried something like F1=2*d(x,t) but seams to not work.

Can anyone please help me?
Thanks :)

Ciprian