Using the idea of forces, explain why the parachutist reaches a terminal velocity and why opening the parachute reduces that terminal velocity.
On leaving the plane the only force acting is weight downwards. As parachutist falls air resistance acts upwards. Resultant force downwards so parachutist accelerates. Opening parachute increases surface area. Opening parachute increases air resistance. Air resistance is greater than weight. Resultant force acts upwards/opposite direction to motion. Parachutist decelerates/slows down.