Below is a video clip featuring a tremendous collision between Desean Jackson of the Philadelphia Eagles and Dunta Robinson of the Atlanta Falcons that takes place early in the 2010 NFL season.  For the discussion that follows, it would benefit the reader greatly if they were to view the video (its only about a minute in length).








Although neither Mr. Jackson nor Mr. Robinson are likely very appreciative of my fascination with this hit, I do sincerely claim that this particular collision makes for a very interesting physics discussion.  What makes it interesting is the fact that both players are very similar in both height, weight, and speed.  Also unusual, as you can see in the video, this particular collision is completely direct, in other words there is no horizontal rotational motion by either player that would otherwise reduce the effects of the applied force during the hit.  You may also have noticed that there is a vertical rotational motion made by Desean Jackson where his torso is violently propelled in the opposite direction that he was traveling in just a half-second before, we will look at this closer a bit later.

According to NFL.com, Desean Jackson is 5' 10'' tall, weighing 175 pounds, and during the scouting combine he completed the 40 yard dash in 4.29 seconds.  Dunta Robinson is listed at 5' 11'' tall, weighing 186 pounds, and he completed the 40 yard dash in 4.34 seconds.  This is enough information to get a rough approximation of the force exerted on Desean Jackson during the collision using Newton's famous second law: Force equals mass times acceleration (F=ma).

First, we can approximate Jackson's speed during the play using his forty yard dash time:

Second, we can find Jackson's mass from his listed weight:

Third, we can find Jackson's acceleration (a.k.a. his change in velocity) if we estimate his velocity immediately after the collision to be approximately 0 m/s.  Dr. Timothy Gay, in his book The Physics of Football, states that the typical tackle has a duration of two-tenths of a second.  Taking into consideration that Jackson's velocity decreases from 8.53 m/s to 0 m/s in 0.2 seconds we have thus found Jackson's acceleration ( or rather deceleration) during the collision:

Finally, now that we have determined Jackson's mass and acceleration, we can determine the force he encountered by simply taking the product of the two:





3400 Newtons is a very impressive amount of force, in fact it's enough to cause Jackson's head and helmet to accelerate at a rate that is over a hundred times the acceleration of gravity.  In an article for Popular Mechanics, Matt Higgins describes a "100-g impact" as fairly commonplace in football, with extreme impacts registering over 150 g's.

  

Newton's Third Law tells us that Desean Jackson applies just as much force to Dunta Robinson as Robinson does to Jackson, which is fairly obvious from the video considering that both players appear semi-conscious at best as they are being helped off of the field.

The following video has an excellent chance to better your understanding of Newton's Third Law and momentum in football.