Falling in Air - Skydiving

To study the effect of air resistance on falling objects, you are going to go skydiving - at least in your imagination. To simplify things, we are going to ignore the horizontal component of motion that you would actually have if you jumped out of a moving airplane - so for this example you won't be a projectile, you will just fall vertically. (Maybe you are jumping out of a hovering helicopter.)

The table below shows several "snapshots" of your motion as you fall. Study the simple numerical analysis of each situation, and use it to gain some conceptual insight into what is happening to you during your descent. The numbers given in the example below are (hopefully) reasonable, but they are "made up" - primarily to make the arithmetic easy. I make no claim to skydiving expertise. So, if you wonder how I knew that the air resistance force is 100N in the second "snapshot" - the answer is, "I made it up!"

Summary:

When you initially jump, the air resistance force on you is essentially zero and you are in free fall. The net force on you equals your weight and your acceleration equals g (10 m/s2).

As you fall your velocity increases. This increased velocity causes the air resistance force on you to increase, which causes the net force on you to decrease. As the net force decreases, your acceleration decreases, so you speed up at a slower rate.

At the point where the air resistance force on you equals your weight, the net force on you will be zero, so you will stop accelerating and fall with constant velocity - your terminal velocity.

As an object falls in air:

• its speed increases.
• the air resistance force on it increases.
• the net force on it decreases.
• its acceleration decreases.

until it reaches terminal velocity, where the net force on it and its acceleration are zero.

last update November 21, 2007 by JL Stanbrough