The Physics of Three Great Catches

In 2008, the video below made the rounds on the internet. Before you start sniping at it, just take a second, chill out, and simply enjoy it.

The clip became such a sensation that MiLB.com had some explaining to do. Even NPR did a story on it. It turned out that the clip was part of an abandoned Gatorade advertising campaign.

There was a lot of internet chatter regarding whether the catch was real or fake. Strangely, most of the naysayers focused on the fact that the catcher is clearly right-handed at the beginning of the video when he squats behind the plate. Yet, just before the replay when he is standing and looking out toward left field with awe, he is clearly left-handed.

It is unclear what the catcher has to do with an outfielder’s ability to climb the wall in this way. Basic physics should be able to tell us if this catch was possible or just some cinemagraphic magic. As Sir Isaac Newton explained well over three hundred years ago, objects do what they do because of the forces that act on them.

One of the key ideas illuminated by Sir Isaac was for every action, there is an equal and oppose reaction. While this idea has been applied to everything from the stock market to politics, it is only strictly true for forces.

Think about a line shot hit toward third base. As the ball interacts with the fielder’s glove, it exerts a large force on said glove, sometimes even causing pain to the hand therein. If you choose to call this the action force, the reaction is the equal force back on the ball causing it to come to rest. Of course, we could have looked at it the other way around – the force slowing the ball as the action, and the force on the glove as the reaction.

Now let’s apply this idea of action and reaction to the outfielder trying to climb the fence. The outfielder digs her cleats into the wall, pushing into the wall and down the wall. Consider these to be the action forces – in toward the wall and down along the wall. The reaction forces then are the wall pushing the fielder away from the wall and upward along the wall as shown below.

The fielder – according to Newton – will do what she does because of the forces acting on her (not the forces she exerts on something else like the wall). The wall will indeed push the fielder upward and will help her go higher than she would without using the wall.

The other force the wall exerts on the fielder is away from the wall. This force will tend to move the fielder back toward the infield. In the video below, you can see Mike Trout use his foot on the wall to bring back a homer. Be sure to notice that he has to actually grab the wall with his hand to create a force toward the wall to counter the force on his foot moving him away from the wall.

Let’s return to the ball girl. If you go back and look carefully, you will see when she uses her foot on the side wall she indeed moves away from the side wall. However, when she pushes with her foot on the outfield wall, she doesn’t move away. Instead, she catches the ball right against the outfield wall.

According to Newton, there must be other forces acting on the ball girl. Indeed there were. She was actually in a harness that helped lift her into position. Thanks to the witchcraft of cinematographers, the harness was rendered invisible in the video.

One of the most amazing and verifiably real catches in baseball history happened on July 11, 1990 – the day after the All-Star Game (yeah, there wasn’t always a day off after the All-Star Game). Well, it wasn’t so much the catch but the interaction with the outfield wall afterward.

In the third inning of the game between Kansas City and Baltimore, Bo Jackson tracked down a drive into the left-field gap, making an impressive catch near the wall. Instead of allowing himself to come to rest by colliding heavily with the wall, he actually ran up the wall. Take a look.

Let’s think through the physics. Each time one of Jackson’s feet touches the wall, he exerts a downward force on the wall and a force into the wall. By action-reaction, the wall pushes him up and away. The effect of the upward force is clearly seen as he moves up the wall.

The easiest way to understand the motion caused by the force away from the wall is to think about the location of Jackson’s belly button with respect to the wall. Why his belly button? The center of mass of a person is pretty close to his belly button.

During his first contact with the wall, Jackson is nearly vertical, so his belly button is relatively close to the wall. When the next foot hits the wall, he is less vertical and more horizontal, so he is a bit further from the wall.

During Bo’s final interaction with the wall, he is nearly horizontal. In other words, he is as far away from the wall as he can be and still remain in contact. This is illustrated in the sketch below.

The red x roughly shows the location of the center-of-mass during each contact with the wall. The vertical blue dashed lines help you see that as he goes up the wall and becomes more horizontal, his center of mass is further from the wall. So, the wall did indeed push him away.

Getting up the wall is really not as remarkable as getting down safely. Jackson’s athleticism is responsible for the subtle forces he had to exert during his last contact with the wall. The reaction to those forces by the wall caused his body to rotate back to vertical just as he landed on the ground.

In summary, I’d like to thank Sir Isaac Newton for providing an even deeper appreciation of the remarkable athletic gifts of major league ballplayers.

References & Resources

• Benjamin Hill, MiLB.com, “Behind the scenes of the Fresno ball girl video”
• NPR, “Foul Ball Catch Too Good to Be True”