What is Free Fall?

To a physicist, the term "free fall" has a different meaning than it does to a skydiver. In physics, free fall is the (one-dimensional) motion of any object under the influence of gravity only - no air resistance or friction effects of any kind, whereas it is air resistance that makes skydiving a hobby rather than a suicide attempt!

You might think that since just about everything we observe falling is falling through the air, that "physics free fall" must be a pretty useless idea in practice. Not so! Any falling object's motion is at least approximately free fall as long as:

• ... it is relatively heavy compared to its size. (Dropping a ball, as in the picture at right, or jumping off a chair, is a free-fall motion, but dropping an unfolded piece of paper, or the motion of a dust particle floating in the air, is not. If you crumble the paper into a "paper wad", however, its motion is approximately free fall.
• ... it falls for a relatively short time. (If you jump off a chair, you are in free fall. After you have jumped out of an airplane and fallen for several seconds, you are not in free fall, since air resistance is now a factor in your motion.)
• ... it is moving relatively slowly. (If you drop a ball or throw it down its motion will be free fall. If you shoot it out of a cannon, its motion won't be free fall.)

You should also note that an object doesn't have to be falling to be in free fall - if you throw a ball upward its motion is still considered to be free fall, since it is moving under the influence of gravity.