1.17 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity
In a free falling object two types of force acts: Drag and Weight. The size of the drag force acting on an object depends on its shape and its speed. If the drag force of an object increase to a point which is equal to Weight, then the acceleration stops. It falls in a constant velocity known as terminal velocity.
Reaching terminal velocity on a parachute:
When a skydiver jumps from a plane at high altitude he will accelerate for a time and eventually reach terminal velocity. When he will open her parachute this will cause a sudden increase in the drag force. At that time drag force will be higher than the weight and he will decelerate for sometime. Later those forces will become equal and reach a new terminal velocity.