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# How to Deconstruct a Football Tackle With Physics

Few things herald Thanksgiving as loudly as turkeys, flight delays, and long hours in front of the TV watching football. It’s an important time in the football calendar, with the playoffs beginning to shape up for both college football and the NFL.

But what about those impacts? I don’t mind watching football, but there’s no way I’m going to take one of those hits. When I see some of these epic tackles, I can’t help but think about physics.

There are some great tools available so that we can analyze the physics of a football hit. Really, we have everything. the masses of individual players? Yup—just search the roster and you can look these things up. Video analysis tools? Again, yes. Personally, I really like Tracker Video Analysis. There’s just two more things we need for a full analysis. I need the video frame rate, but this is trivial. Although some impacts are replayed in slow motion, they are also shown in real time. What about a distance scale? Oh wait! It’s right there on the field with the yard lines. We are all set.

Let’s start with a collision. I just did a search for “biggest football hits” and quickly found one that would work. In this case, I am going to look at the Clemson vs. Syracuse game from 2017. The play has Clemson wide receiver Trevion Thompson (205 lbs) tackled by Parris Bennett (216 lbs) from Syracuse.

I like this collision for two reasons. First, the motion is mostly up and down the field and not side to side. This means that the camera gets a side view of the motion and they line up with the yard lines for easy measurement. Second, Bennett actually lifts Thompson up and pushes him back. That looks cool.

Now, there is one small problem. The camera zooms in and pans. This means that the location of the origin changes with respect to the view of the camera and the pixel-to-meter ratio changes. Fortunately Tracker Video Analysis has a nice method to account for this camera motion—the calibration point pairs. Basically, you set the scale of the video and then track two points on the background. The app then adjusts the virtual coordinate system so you can mark the real locations of the two players. Here is the resulting view from that transformation.

Next, I just need to mark the location of both players in each frame. Here’s what I get.

Since these are position-time plots, the slope of the line will give the velocity of the player. Here is what I get. I’m using units of meters per second because I’m not a barbarian.

• Initial velocity for Bennett = 6.05 m/s
• Initial velocity for Thompson = -1.33 m/s
• Final velocity for Bennett and Thompson = 3.03 m/s

Great. But what can we do with these values? The answer is “more physics.” Let’s start with the momentum principle. This says that the total force on an object (or football player) is equal to the rate of change of momentum. But what is momentum? It’s the product of mass and velocity (represented by the symbol p). In fact, I can write the momentum principle in terms of the change in momentum like this: