MadSci Network: Physics

Re: How do I measure football cleet traction?

Date: Fri Jan 20 11:17:45 2006
Posted By: Tom Cull, Senior Staff Scientist
Area of science: Physics
ID: 1137286025.Ph

This seems like a fun experiment to perform.  The problem might be getting 
equipment (like different shoes) without spending a few bucks.

For background, I recommend the book, "The Physics of Football" by Timothy 
Gay.  Dr. Gay is a physics professor at the University of Nebraska-Lincoln 
and a football fan.  Chapter two of his book has some very good and simple 
analyses of the physics involved in the battle between offensive and 
defensive line in the trenches (he calls it the pit).  He also talks 
specifically about cleats and traction in another section of his book.

I define traction as the friction or grab generated by the shoes with the 
playing surface. Friction is highest between two rough surfaces.  Cleats 
are a rough surface because they make an uneven surface on the bottom of 
the shoe.  In fact, the cleats actually dig into the grass which means the 
traction or drive of the athlete is in part determined by the strength of 
the cleats and the lateral strength of the playing field.  We might expect 
longer cleats to dig into the ground deeper and provide more traction on a 
slightly damp field, but maybe not on a rock hard dried out field, and 
definitely not on an icy surface or artificial surface.  

Consider the case of a blocking sled of known weight (or add weight of other 
football players) being pushed by one or more football players 
(blockers).  To move the sled by pushing (blocking), the blocker needs to 
generate enough force to overcome the static friction between the sled and 
the playing surface.  To keep the sled moving at a constant speed, the 
blocker needs to generate enough force to balance the kinetic friction 
between the sled and the ground.  The blocker generates force by pushing 
his feet against the ground.  Better traction between the blockers shoes 
and the ground should yield more force.

So if you can get access to a block sled and a few friends and several 
different styles of cleated shoes you can run an experiment of your own. 
This will really seem like a test of strength or a blocking drill :) 

Your lab area would be a space of grass with distance markings on it (a 
football field might work nicely).  Get a stop watch.  Have the test 
blockers take turns pushing the sled through say five or ten yards and 
time how long it takes them.  They can try this several times for each 
pair of cleats you want to test.  You might want to randomize shoe order 
because people are invariably going to get tired doing this exercise.  

All things being equal (which is a huge simplifying assumption), the 
cleats with better traction should produce the fastest times because the 
blocker can generate more force and get better acceleration and top-end 

How detailed is up to your available time and careful application of 
experimental control.  You can probably come up with a few better ways to 
do this test.  The key is to run multiple averages with each pair of shoes 
and probably over different field conditions.

I hope this helps,

Tom "Option Quarterback" Cull

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