The arrow opposite the direction of travel is the air resistance or drag.
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If d = 16*t^2 then there is no significant air resistance.
Well, the more the air resistance, the lower the acceleration.
Air speed or ground speed?
Terminal Velocity. This is the velocity at which the accelaration from Earth's gravity and the drag from air resistance reaches equillibrium.
It is a force which acts in the upward direction.