1.17 Describe the forces acting on falling objects and explain why falling objects reach a terminal velocity

When an object begins to fall, it accelerates (gains speed) because its weight (the downwards force) is greater than the drag (air resistance). 

As it accelerates, the drag also increases until the forces balance each other out. The resultant force then equals zero (because they're balanced) and in order for an object to accelerate, forces must be unbalanced, so due to this, the object can no longer accelerate. It reaches the maximum velocity that could possibly be achieved during free fall: terminal velocity.