Explain what terminal velocity means.

Consider an object in free fall through the atmosphere. Firstly, we need to consider the forces acting on the object. A force diagram will help here. Weight acts from the centre of mass of the object towards the Earth and equals the object's mass times the gravitational field strength of the Earth. Air resistance acts in the opposite direction to the direction of motion of the object which is this case is towards Earth. Initially as the object is released, the weight force is much larger than air resistance since the object begins stationary. There is a resultant force towards the Earth and due to Newton's 2nd Law, F = ma, the object will accelerate towards Earth. As the velocity increases, air resistance increases such that the net force decreases and the acceleration decreases. The air resistance will increase to equal the weight force, resulting in no net force and zero acceleration. The velocity at which this occurs is the called the terminal velocity, and the object will continue to travel at this velocity unless any external force acts.