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Air Resistance and Skydiving - help5

As a skydiver falls, there are changes in the speed at which she falls. These changes in speed lead to changes in the amount of air resistance. This in turn changes the net force and the acceleration experienced by the object. These changes continue to occur until the objet reaches a terminal velocity.

There are many nearly-identical versions of this question; they differ in terms of the ordering of the answer options. Here is one of those versions:

Question 5

A skydiver is dropped out of an airplane at an altitude of 10000 feet. She reaches a terminal velocity 60 seconds later. Consider four positions during her fall.
A: Initial State (t = 0 seconds)
B: 15 Seconds After Drop
C: 45 seconds After Drop
D: 60 seconds After Drop
Toggle through the set of vector diagrams at the right to identify the relative magnitude of the velocity vector for each of these four positions. (Consider vertical motion only.)

The motion of objects is influenced by the forces that act upon it. For a falling skydiver, the two obvious forces are the gravity force and the air resistance force. Gravity acts downwards and will undergo little change over the course of the fall. On the other hand, air resistance acts upwards, countering gravity. And since its value depends upon speed, it will change as the skydiver increases its speed.

Initially (at t=0.001 seconds), the velocity is 0 m/s and the air resistance is essentially 0 N. This results in an unbalanced force that causes an increase in the velocity of the skydiver. As the skydiver speeds up, there is increase in air resistance. Since air resistance counteracts gravity, the increase in air resistance results in less net force. But since there still is an unbalanced force, the skydiver's velocity keeps increasing. In fact, as the skydiver continues to fall, the velocity continues to increase, resulting in an increase in air resistance and a decrease in the amount of unbalanced force (F_net).

Eventually, the air resistance force becomes as big as the gravity force and the forces are balanced. At this point, there is no acceleration ... no more speeding up ... no more increase in air resistance ... and a continuation of the zero net force. We refer to this state of motion as the terminal velocity state. This is the largest velocity that the skydiver ever experiences.

You can learn more about air resistance and skydiving by using the following link to The Physics Classroom's Tutorial section.

Free Fall and Air Resistance