State Newton's 2nd Law, and use this explain the vertical motion of a parachutist in the descent from her plane to Earth.
Newton's second law: Fnet = m.a. The net force (vector) on an object is directly proportional to the product of its mass (scalar) and its acceleration (vector). At the instant the parachutist leaves the plane, there is a downward force, F = mg ~ 10m on her. (This force is termed 'weight'). There is no upward force as of this instant. The net force downwards causes the downward acceleration of the parachutist, as deduced from Newton's 2nd Law. As the parachutist gains velocity, however, drag force plays an increasingly significant role (as Fdrag is proportional to A x v2, where A is the cross-sectional area and v is the velocity of the object). The drag foce acts in the opposite direction to the parachutist's weight, and therefore the net force diminishes with increasing downward velocity. Ultimately, the drag force and her weight are equal and opposite forces, so that Fnet = 0. This implies that she is no longer accelerating and is falling at a constant velocity, v1. When the parachutist opens her parachute, effect is that the cross-sectional area of the falling body has increased significantly. The drag force therefore increases significantly (because as stated above,Fdrag is proportional to A), and Fnet acts upwards, leading to a downward deceleration (or upward acceleration) of the parachutist in her descent. The velocity of the parachutist begins to decrease, until the drag force and her weight have equalised once more. At this stage, the parachutist continues to fall at a lower, constant velocity, v2, until she reaches the ground.
