A skydiver exits the airplane with his parachute on. He falls at a terminal velocity of about 120mph. He opens the parachute (note that his weight did not change, but his cross sectional density is drastically decreased) and now falls at about 17mph (I take my figures from an old Switlick silk parachute from ww2). The cross sectional density of tandem jumpers is not quite the sum of the two weights, as the air flows around the lower jumper and impinges on both to some extent, but the net result is a higher csd. In parachute training, they sort the jumpers by size as much as possible when doing a mass jump, heaviest out the door first, to avoid the big heavy guys from landing on and collapsing he long floating light guys' chutes on the way down.
Air resistance gets in the way of a lot of newtonian physics. This is one case. Artillery ballistics is another. The fall of shot can not be predicted by newtonian physics if you are not in a vacuum. The math is very hairy. The Navy invented mechanical analog computers using cams and rollers to solve the problem. Look up Siacci and Vernet.
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