Using IR Technology to Illuminate Some Interesting Physics

Game #1 of the 2011 World Series marked the debut of the "Fox Hot Spot", in which an infrared (IR) camera was used to produce a "heat map" as its image. With such a camera, bright spots are hot and dark spots are cool, relatively speaking. As it turns out, in the 9th inning of that game, an event occurred that showed the potential value of this technology. Adrian Beltre hit a ball nearly straight down into the batter's box; it subsequently bounced to 3B and Beltre was thrown out. Both Beltre and the announcers thought the ball actually hit Beltre in the foot, which would have been a foul ball. The umpire didn't see it that way and Beltre was ruled out on the ground ball.

IR Images of Beltre Foul Ball.

Now take a look at this video, including the IR clip shown at the right. There is a lot of interesting physics in the IR clip. As Yogi once pointed out, you can observe a lot by watching. So watch the clip, then return here for some explanations of what you saw. For convenience, I am displaying a sequence of six screenshots in the pictures below.

1. Note how the ball heats up upon contact. That is largely due to the friction of the strands of yarn rubbing together as the ball compresses, then expands back to its normal shape during the collision. That process is inherently inefficient and much of the initial energy is dissipated by the friction, resulting in the interior of the ball heating up. That effect is clearly visible with the IR camera. The loss of energy due to the heating up of the ball is what results in a "coefficient of restitution" (COR) of the ball that is less than 1 (more like 0.5, meaning about 75% of the available energy is lost). The COR characterizes the "bounciness" of the ball. In fact, a baseball is not very bouncy. If a baseball were a superball, with a COR close to 1, then we probably would not have seen the ball light up nearly as much. But, read on for another reason for the ball to light up.

2. Note that the region of the bat near the impact point also heats up. That is due to the sliding friction between the surfaces of the ball and bat. The bat hit the ball with a glancing blow (which is why the ball went nearly straight down), so the ball did a lot of sliding along the surface. The friction from that sliding heated up the surface of the bat as well as the surface of the ball. The heating of the ball due to the COR is a "volume" effect whereas the heating due to the frictional sliding along the bat is a "surface" effect. I speculate that had the bat hit the ball with a more nearly head-on collision--as opposed to a glancing one--we would not have seen the bat light up nearly as much. Some of you may recall the pre-game ceremonies at the 1999 All Star Game in which Ted Williams asked Mark McGuire if he had ever smelled smoke after hitting a foul ball. The foul ball comes from a glancing collision and a glancing collision means friction and heat--hence the smoke. Ted was not a physicist, but he understood these things very well!

3. Now look at the tip of Beltre's shoe, which also lights up, indicating that the ball did actually hit the shoe. The lighting up is once again due to the sliding friction between the ball and the tip of the shoe. As the article points out, the IR image clearly shows that the umpire got it wrong. So, that's a lot of interesting physics in such a short clip.

The six images are below, running sequentially from top to bottom. The first image occurs prior to ball meeting bat. In subsequent images, you can see the contact region of the bat lit up. You can also track the initial downward trajectory of the ball, which is blurry since the ball is moving rapidly. In the next-to-last image, the ball is clearly visible but not blurred, suggesting that the ball has impacted something (the ground? the shoe?), slowing it down considerably. To me it looks like it is in contact with the ground, since you can see the region below the ball lit up. I don't understand why the ball is not also lit up. It is not possible from this image to know whether or not the ball also hit the tip of the shoe, which is blocked from our view by the ball. However, in the last image, the tip of Beltre's left shoe is barely lit, while the ball has now bounced upward. Since the image is less blurry than the earlier ones, the ball has lost a lot of the speed it had just after leaving the bat. Moreover, if you look very carefully, you can see the region just in front of the shoe, presumably on the ground, is also lit up, suggesting that the ball hit both the ground and the tip of the shoe more or less simulataneously.

See also Infrared Cameras Debut in Baseball Telecast for World Series, a very nice article by John Matson appearing in the on-line edition of Scientific American, October 20, 2011.

I hope my readers appreciate the title of this page.