Competency #1
Answer and Explanation
By definition, gravity is the only force acting on an object in free-fall.
Whenever air is present, there will be air resistance acting on a moving object. However, for some objects at certain speeds, the air resistance is insignificant and the object is essentially in free-fall. For example, if you drop a baseball and a crumpled piece of paper from 5 feet, both objects will hit the floor at the same time. Air resistance is acting on both objects, but is too small to have a noticeable effect. However, dropping the baseball and paper ball from 500 feet would result in greater speeds, significant air resistance, and would not be a free-fall situation.
For an object at terminal velocity, gravity and air resistance are acting equally on the object.
Since the forces are equal and pulling in opposite directions, the net force on an object at terminal velocity is zero. Since the net force is zero, the acceleration of the object must be zero (by Newton's 2nd Law). If the object is not accelerating, the object must be traveling at a constant velocity. This constant velocity is known as the terminal velocity.