How hard does an NFL defender hit?

As an avid football fan, I spend every weekend watching hours of NFL action. Any football fan will tell you they love to see the big hits. I’m no different. In watching this weekend’s games, I found myself wondering exactly how hard do they hit and how fast are these athletes moving when they deliver these devastating blows? I did some research on average weight and speed stats for various positions for athletes performing at the NFL combine and used these in our kinematic and Newtonian equations to find out for myself. (Note, for ease of calculation, I’ve ignored wind resistance and assumed constant acceleration. All calculations are completed assuming 40yards of running distance.)

Cornerbacks: While not the biggest guys on the field, corners make up for their lack in size with blazing speed. Average 40yd times for cornerbacks in the past 5 years at the combine clock in at 4.55seconds. Using the kinematic equation Δx=V_°t+½at² (again assuming constant acceleration) I found that the average NFL corner can accelerate at 3.56m/s². Using the equation V=V_°+at I found that at the end of the 40 yards, corners are running at an average speed of 16.2m/s or 36.2mi/hr. With an average mass of 87.5kg, these corners can deliver a force of impact of 312N (after a 40yard running start, that is) (using the equation F=ma).

Linebackers: These big boys rely on their size to punish running backs and receivers unlucky enough to find themselves in the way of the oncoming pain train. Average 40yd times for linebackers in the past 5 years at the combine clock in at 4.77seconds. Using the same equations as above, I found that the average NFL linebacker can accelerate at 3.25m/s². At the end of the 40yds, ‘backers are traveling at an average speed of 15.5m/s or 34.7mi/hr. With an average mass of 109kg, ‘backers plow into the offense with a force of 354N.

Safeties: The last line of defense. These gifted athletes averaged a 40yd time of 4.62seconds over the past 5 years at the combine. Using this time, I found that safeties can accelerate at 3.46m/s². At the end of the 40yds, safeties are traveling an average speed of 16.0m/s or 35.8mi/hr. With an average mass of 94.8kg, safeties run over the competition with a force of 328N.

Needless to say, I don’t think I’d want to be on the wrong end of an NFL defender moving at full speed.

Static vs. Kinetic Friction

I was watching Top Gear this weekend (obviously the UK version) and thought about static versus kinetic friction in terms of a drag race and why static friction coefficients are greater than kinetic friction coefficients.  The hosts often test to see how fast a car can go in a given distance.  When they first step down on the gas peddle, there is a significant amount of static friction preventing the car from moving immediately.  Enough force must be applied to get the car to move to overcome the molecular interactions of the tires and ground "locking" together.  Once the car is moving, these forces are easier to overcome because the car already moving and has acceleration, forces being applied, etc.  This means there is less kinetic friction to overcome because the molecular interactions aren't as "locked" together.  So once the car is moving, it speeds up really quickly.  So the coefficient of kinetic friction is less than the coefficient of static friction.