Why physics matter in a tackle

Bone-jarring tackles almost always make the highlight reel in a National Football League game. And those tackles happen more than 100 times per game. 

They may not always look pretty; a big linebacker engulfing a smaller running back. But the play is actually employing some pretty elegant physics, including p=m*v. 

To put another way, the momentum of the Cleveland Browns’ linebacker equals his mass times his velocity. You can figure out the same momentum for, lets say, the New England Patriots running back. When it’s applied, and the momentum of the Browns’ linebacker exceeds the Patriots' running back, the runner is stopped. 

As you can imagine, as players get bigger and stronger, the numbers in the equations grow larger. 

Fortunately, players can also use physics to cushion the impact. 

Football coaches like to tell players to stay low and keep the head up while tackling. This might be a good technique (you can see what you are doing), but it’s also good physics. 

That’s because by keeping the feet planted and the head up, the player can direct the force of the impact into their equipment or into the ground. 

And directing, or controlling the force of a tackle is just as important for the Browns’ linebacker as it is for the Patriots' running back. That's because when players collide, the force of the impact is distributed equally between them. You can thank Sir Issac Newton, who discovered the law but had never heard of football, way back in the 17th century. 

Newton's third law of motion says if two objects interact, they exert opposite and equal forces on each other. 

So if the Browns’ linebacker has a momentum of +10 and the Patriot’s receiver catches the ball but is standing still looking for a place to run, he has a momentum of zero. After they hit each other the linebacker’s momentum is +5 and the receiver’s momentum is +5. But the bigger player will determine where they end up because his mass is greater. 

Which is why good players learn to absorb hits in the right places. No matter where the hit is absorbed the total force to the player is the same, so it makes sense to absorb the hit in the shoulder pads and the ground, rather than the ribs.

It turns out there's a science to even the most basic football techniques such as blocking and tackling. And when done properly they can prevent injury and even improve a player's game. 


—Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog! 

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