if you need to find the force, then yes you use F = ma
and yes the acceleration is
the second derviative, even in three dimensions!
(what is worrying you about that?
yes, that does give you the total
but W = F*s only works if F is constant
here, F isn't
constant, so you would have to use W = ∫ F.ds
however it might be simpler to use the work-energy theorem, and calculate the difference in kinetic energy