> >You could not "let the truck" leave you behind by jumping straight up because you are moving at the same speed with nothing to slow you down because you eliminated the air resistance.
>
> [I ask this hesitantly, having been out of physics class for several years, but]
> Wouldn't the truck's thrust still have to be accounted for, in comparison to your lack of continual thrust as soon as you leap from the truck, or would removing air resistance be enough to compensate for this?
>
> Penny*smiles with timid uncertainty*stamp

No, the question was asked assuming that the car is moving at a constant speed of 70 mph (with things like friction from the road surface accounted for). So we can assume that, while there is no air friction, the truck's engine is being used only enough to work against any friction from the ground -- it's not accelerating.

Both you and the truck are moving at 70 mph, and since you have nothing to slow you down when you jump, and since the truck is working only enough to maintain a constant speed, you both keep the same speed. Remember, without any sort of counter-acting, you will never stop moving once you start.

Stephen