The mass of the skydiver-with-parachute is unchanged by the opening of the parachute, so the gravitational force must be the same before and after the parachute is opened. Apparently a new force acting on the falling entity has suddenly been introduced, a force which depends on the new configuration.
In this module, simple differential equations are used to study the effects of the "drag" force which slows the motion of a body moving through a fluid (in this case: air) and are responsible for the safe landing of the skydiver.