Howstuffworks.com's "How the Physics of Football Works"

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How the Physics of Football Works
by Debbie Selinsky

Watching a Sunday afternoon football game could be teaching you something other than who threw the most passes or gained the most yards. Football provides some great examples of the basic concepts of physics! Physics is present in the flight of the ball, the motion of the players and the force of the tackles. And according to Professor David Haase of North Carolina State University, the more you understand the science, the better you understand the game.

In this edition of How Stuff Works, with Haase's help, we'll look at the meaning of terms used in physics -- mass, force, momentum, torque, velocity -- and how they apply to the game of football!

Photo courtesy of North Carolina State University
Do these guys look like they're studying physics?

What Exactly Is Physics and What Does It Have to Do with Sports?
A good way to start to learn about physics is to think about something we all know. For example, when you throw a ball across the yard to your friend, you are using physics. You make adjustments for all the factors, such as distance and wind and even how heavy the ball is. The farther away your friend is, the harder you have to throw the ball. This physical adjustment is done in your head. It's physics -- you just don't call it that because it comes so naturally. If there were no set laws, the ball would behave unpredictably when you throw it and you wouldn't be able to adjust your throw.

Haase tells his students that the universe is made up of matter. If that matter sits there and does nothing, it's not very interesting. With physics, that matter goes into action and does a lot -- that's when things get interesting! "A game of football is one of the best, most interesting science lessons you'll ever see," he says. "How far a quarterback's pass goes, how much hang time a punt has, how hard a lineman's hit is -- all these and more are governed by the laws of physics."

Haase has observed that coaches and players actually know a lot more about physics than they think. However, this knowledge appears to be instinctive; players and coaches don't consciously translate the mechanics of sports into the basics of physics. But when that translation is made, we understand and appreciate even more how amazing some of the physical feats on the football field (and the baseball diamond, the basketball court and the tennis court) really are!

Relating football and other sports to the science of physics leads to better and safer equipment, affects the rules of the sport, improves athletic performance, increases appreciation of the sport and is good public relations for physics, a science not always thought of as a lot of fun, Haase says.

The Language of Physics
Let's take a quick look at some of the most important physics concepts that we'll encounter in our discussion of football and physics:

Acceleration -- the rate of change of velocity with respect to time (This is calculated by subtracting the starting velocity from the final velocity and dividing the difference by the time required to reach that velocity.)
Force -- the influence on a body that causes it to accelerate
Mass -- the amount of substance that an object contains
Momentum -- the mathematical product of the mass of a moving object and its velocity
Inertia -- the resistance of a moving object to a change in its speed or direction, and the resistance of a stationary object to being moved
Torque -- a force that tends to produce rotation or twisting
Velocity -- the speed with which an object travels over a specific distance during a measured amount of time
Center of mass -- the area of mass that exerts maximum force

Now, let's look at lessons every football coach gives his players and then look at Haase's physics "translation":

Coach's instructions: Hit and tackle low. (Physics translation: Hit below the other player's center of mass so you knock him over more easily.)
Coach's instructions: Don't arm tackle. (Physics translation: Tackle by using force exerted through your own center of mass -- that's the way to exert maximum force.)
Coach's instructions: Keep your legs pumping. (Physics translation: Keep pushing on the ground to exert force and to maintain your balance.)
Coach's instructions: Use leverage to block. (Physics translation: It's a lot easier to turn someone than to move them. A good defensive lineman often turns a blocking offensive tackle so the defensive player can get around him. Reggie White did this best, Haase says!)

Here's another example -- one Haase gives in fun -- of that translation: A football announcer might describe a large offensive lineman this way: heavy, wide body. What this really means, Haase says: "Large mass, low center of gravity and large moment of inertia."

What Are Some Other Examples of Physics at Work in Football?
Since physics is a quantitative sport, developing some units and measures is a good way to begin to understand the effects of physics on football. Consider these useful numbers and units developed by Haase:

A player at full speed ~ (roughly equivalent to) 22 miles per hour (22 mph = 9.8 m/s ~ 35 kmph)
A linebacker ~ 220 pounds (98 kg)
An offensive lineman ~ 300 pounds (133 kg)
A good punt = 4 seconds and 40 yards (40 yards = 36.6 meters)
A long pass = 40 yards in the air
One (Ronnie) Lott (a hard-hitting safety for the San Francisco 49ers) = the energy and momentum of a 220 pound safety at full speed (220 pounds * 22 mph = 960 kg-m/sec)

Keep these measurements in mind as you read some of Haase's observations and calculations (these were the basis of a 1998 story in ESPN Magazine):

If you were caught between two 220-pound players colliding full-speed in midfield, the impact on your body would be roughly equivalent to trying to catch a bowling ball dropped from the 13th floor of a building! (See illustration at right!)
Despite the difference in their sizes, a 210-pound linebacker can often take someone down with the same force as a 260-pound lineman. Why? Momentum, the product of mass (body size) and velocity (speed). A lineman doesn't usually have much time or space to build up speed before he makes a tackle. This means he has to rely on mass to stop the ball carrier. A linebacker, however, has more time and space to build up velocity, so he can achieve the same momentum as the big guy without his size.
Taking a hit from a 250-pound linebacker moving at about 20 mph would feel something like falling off the top of a 15-foot ladder in protective pads. (See illustration at right!)
If a wide receiver ran for the goal posts on every offensive play, he would cover a distance of only 1 3/4 miles by the end of the game.
Players try to pass the ball in an overhand motion that creates a spiral, a gyroscopic torque that makes the football more stable in flight and helps it go further with greater accuracy.
The best punts hang in the air for 3 or 4 seconds, which is long enough for the punter's teammates to get downfield near the receiver. This "hang time" is determined by how high the punter is able to lift the ball. For a punt to travel 50 yards in no less than 4 seconds, the punter must lift the ball 65 feet into the air (about five stories high). In order to do that, the football must leave his toe at 50 mph.

What Makes a Tackle or Block Destructive?
The rule of thumb in football is that a player is tackled or blocked by momentum, but injury is caused by the kinetic energy lost in the collision. The following factors determine the damage done in a collision on the field:

The energy lost in the collision -- Lost energy or the energy distributed to each participant is important. Energy = (1/2)mv^2, so a massive lineman or quicker but smaller linebacker can each have his effect.
Where the energy is deposited -- Certain parts of the body, including the back, the head and neck and the knees, are more susceptible to damage. How the force of the collision is spread also contributes to the severity of injuries.
Preparedness -- If a player is braced for a hit, he moves to reduce the hit's effects but he also braces the muscles that connect his head, torso, etc. This means he becomes more like a single, rigid object, which is more likely to bounce and absorb less energy in strained ligaments. If a player is hit unexpectedly, he is more vulnerable to a variety of injuries throughout his body.
Use of "gravity" as the second tackler -- A good tackler uses gravity to do the tackling. This is why he hits low and uses gravity to pull the ball carrier over. Sometimes a high hit has the same effect.
Effect of the ground -- Astroturf is essentially an asphalt parking lot covered with carpet. It is not energy absorbing and it pins the player's shoe cleats firmly to a particular spot. If he's hit hard and his foot stays put, this puts tremendous force on the ankle or knee and can result in a potentially career-ending injury to the anterior cruciate ligament, Haase says.

Football players wear pads to reduce the chance of injury. But the soft cushioning is not all there is to pads. The real purpose of the padding is to absorb energy. So pads have a soft part (on the inside next to the player's body) to absorb energy and a hard outer cover to spread the force of collisions over a much larger area around the point of impact.

Where Can I Learn More About Sports and Physics?
If you'd like to meet Haase, who is director of NCSU's Science House, you can attend the annual summer Burroughs Wellcome Science of Sports Camp on the NCSU campus in Raleigh, N.C. It's open to K-12 students and educators.

If you live in the San Francisco area, you can participate in some of the Exploratorium's sports science activities and visit their Sport! Science exhibit.

If you've aged out of some of these activities but would still like to learn more, here are some good books on the subject:

"The Sporting Life" by Susan Davis and Sally Stephens (1997, Exploratorium) (Haase uses this book in his summer camps!)
"The Physics of Sports" by Angelo Armenti Jr. (1992, American Institute of Physics Press)
"Sports" by Julian Rowe [Science Encounters Series] (1997, Rigby Interactive Library)
"Sportscience: Physical Laws and Optimum Performance" by Peter Brancazio (1984, New York: Simon & Schuster)

Links

Special Thanks to Dr. David Haase!

Louisiana native David Haase is a professor of physics and director of the Science House at North Carolina State University in Raleigh, N.C. He is an expert on sports science and has been featured in articles in many publications, including The New York Times, The Seattle Times and ESPN Magazine. He heads up a summer Sports Science Camp for teachers and young people K-12 on the NCSU campus. He conducts research in nuclear physics and has measured nuclei that are shaped like footballs.

About the Author
Debbie Selinsky is senior editor at How Stuff Works.

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