Well, then … how do they do it?
Catching a Ball the Human Way
Some people call this the Chapman strategy. It’s from a 1968 paper by Seville Chapman in the American Journal of Physics titled “Catching a Baseball.” Chapman’s idea is that the outfielder will see the ball in the air and then move in a manner such that the apparent position of the ball (with respect to the player) has a constant velocity. This is also called the optical acceleration cancellation (OAC) method. (See: “Catching Fly Balls: A Simulation Study of the Chapman Strategy.”)
But what does OAC really mean? It means that a real-life player is relying on their eyes to figure out where the ball is in relation to them, how quickly it’s moving, and whether they’ll need to back up, scoot forward, or stay put to catch it.
Suppose you are a player watching a fly ball. Now you take out a ruler (I recommend one with metric units) and hold it upright by one end with your arm stretched out horizontally, the way you would hold up a cross to ward off a vampire or use a hand mirror to see your face. At first, suppose the ball appears to be lined up with the 8-centimeter mark on the ruler. An instant later, it appears to have moved upward and be at the 10 centimeter mark. This reading on the ruler is the apparent position of the ball. It’s related to the ball’s angle above the horizon and not its actual distance from you.
Velocity is defined as the rate of change of position, so if you keep measuring this apparent position at different times, you can get an apparent velocity. Just as the velocity tells you how fast the ball’s position changes, acceleration is how fast the velocity changes. Looking at the change in apparent velocity will give the apparent acceleration (the optical acceleration). Yes, I know that seems like a lot to do—and you don’t actually have to do it. Humans can estimate this apparent position and acceleration fairly easily simply by looking at a moving object.
Illustration: Rhett Allain
What would happen if you were to plot this apparent position (I’m using the variable ya) as a function of time? Here is what that would look like for three different fly balls. One of these will land short of the player, one will go right to the player, and one will go over the player’s head.