U of M News Wire: September 13, 2007

 
Physics is one reason the I-35W bridge collapse casualties count wasn't higher

By Deane Morrison
U of M News Wire

When the I-35W bridge fell on Aug. 1, several cars rode the bridge down and came to rest on pieces of pavement in the middle of the Mississippi River. The people who emerged from their cars could count themselves lucky; they had just survived a 60-foot drop. Besides sheer luck, one reason they made it was the way the laws of physics worked in their favor, said University of Minnesota physics professor James Kakalios.
 
“According to the [video] tapes, the bridge took four seconds to fall,” Kakalios said. Normally, a 60-foot drop would take close to two seconds. The extra time came from the crumbling of the bridge supports. Or, as Kakalios explained it, that extra time came from the redistribution of energy.
 
Consider a bridge the size of I-35W falling freely. At rest, it would have a great deal of potential energy; as it fell, it would convert the potential energy to kinetic energy -- the energy of motion. The farther it fell, the faster it would go, the more energy it would have, the greater the force needed to slow it down or stop it, and the greater the impact it would have when it finally stopped.
 
When the I-35W bridge fell, the steel girders under the middle part resisted the fall. As they crumbled, they absorbed energy from the falling concrete bridge deck, slowing it down.
 
The energy of the falling bridge also went into pushing a large volume of air out of the way. The energy to set the air in motion came from the kinetic energy of the plummeting structure, and overcoming this air resistance may not have been a trivial matter.
 
“In autos, 25 to 60 percent of the energy of gasoline is used to fight air resistance,” said Kakalios. “Without air resistance, fuel economy would be much higher.”
 
A note on kinetic energy
 
Because an object’s kinetic energy rises in proportion to the square of its speed, doubling the speed gives the object four times as much kinetic energy. Conversely, cutting the speed by half cuts the kinetic energy down to a quarter of what it would have been. If the falling bridge was indeed slowed from 40 to 20 mph, it's easy to see how the reduction in energy available for doing damage may well have saved lives.
 
Thanks to all this resistance, the bridge delivered less of an impact to its load of cars and their occupants. In free fall, it would have been traveling about 40 mph when it hit the water. But based on the time it took to fall, it probably hit at about 20 mph, Kakalios said.
 
Physics to the rescue

The police, firefighters, medical professionals, divers and others who helped survivors and searched for victims are examples of the “everyday heroes” who make use of modern technology.
 
In his new U of M freshman seminar class on “The physics of everyday heroes” this fall, Kakalios, known for his popular seminar on the physics of superheroes, explains the science behind technologies that help real heroes. Topics include infrared heat-sensing to find survivors of fires, MRI scans, and possible future technologies such as "functional MRI" scans to sense a person's thoughts.
 
“The physics that drives all these instruments is quantum mechanics, the modern theory of atoms and light,” Kakalios said. It's all based on two technologies: lasers and transistor-based semiconductors, which also make cell phones, laptops, DVDs and similar devices possible.
 
In other words, Kakalios said, “all the technologies without which my teenage children would not find life worth living.”
 
 
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U of M researchers launch public Web site for monitoring trends in Internet traffic
 
By Mark Cassutt
U of M News Wire

University of Minnesota researchers have built special tools and a public Web site that will provide up-to-date information about current Internet growth and monitor trends in Internet traffic. The project, called MINTS (Minnesota INternet Traffic Studies) -- www.dtc.umn.edu/mints/ -- is a system that monitors Internet traffic reports from more than 100 sites, including some of the world's largest traffic exchanges. In addition to statistical analyses and evaluations, live links are provided to the original traffic reports, allowing the public to check for themselves what is happening on the Internet.
 
According to University of Minnesota researcher Andrew Odlyzko, current Internet traffic growth is slowing down -- about 50 percent per year -- which is contrary to the commonly held belief that overwhelming floods of data are clogging the Internet and threatening to collapse the network.
 
Is the Internet in danger of collapsing under the “exafloods” of video and P2P (peer-to-peer) traffic? Do service providers need to throttle users and prevent network neutrality in order to provide acceptable service at reasonable cost?
 
“There are many claims to that effect, but very little hard data,” said Odlyzko. “In particular, there have been no good statistics on just how fast Internet traffic is growing. MINTS should provide warning to the public and to decision makers if there is a sudden change in demand for Internet transmission that might call for drastic action. This has important implications for technology development as well as public policy.”
 
The development of the MINTS software was carried out by graduate students Anuraag Pakanati and Sungil Hong, under the direction of Odlyzko, an ADC professor and director of the University's Digital Technology Center. Odlyzko was also the first person, a decade ago, to debunk the myth of "Internet traffic doubling every 100 days" that did so much to inflate the Internet bubble.
 
MINTS research has been supported by the University's Digital Technology Center, and the U of M's Minnesota Supercomputing Institute.
 

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Growing Concerns
A parenting question-and-answer column with Dr. Martha Erickson of the University of Minnesota
 
 
Question: Recently a neighbor told me she saw my 15-year-old son and his friend drinking beer. I’m furious with my son but I'm not at all sure what to say to him. I know he’ll be very upset that the neighbor was such a snitch. How should I handle this?

Answer: You are fortunate your neighbor cared enough to tell you about what she saw. And, although he may not realize it yet, your son is fortunate to have adults looking out for his well being. Despite the fact that teen drinking is illegal, most teenagers report that they experiment with alcohol. But how parents handle it can make a big difference in years to come. Here are a few suggestions for how to use this situation as a teachable moment for your son, helping him develop the tools to see him safely through this challenging stage of life.
 
• Without overreacting, tell your son what you learned from the neighbor. Let him know you are glad the neighbor cared enough to tell you.
 
• Encourage your son to talk with you about where he got the beer, why he chose to drink and how he felt about it. If you can listen to his story without flying off the handle, you are laying the foundation for the important steps that follow.
 
• Let him know you understand the forces that encourage young people to drink: peer pressure, advertising and the expectation that, at least in the short run, it will feel good. Brainstorm with your son -- or even role-play, if h’s willing -- about how to resist that pressure and still be “cool,” which is important to teens.
 
• Then tell him simply, clearly and firmly your rules about underage drinking. Remind him of the risks of drinking (for example, it's illegal, it leads to physical risk and it impairs judgment), and tell him what consequences you will impose if you learn he has been drinking again.
 
• Join with other parents to set community standards for teen drinking and agree on how you all will share responsibility for enforcing those standards. Many middle schools and high schools help organize parent networks like this. It simplifies life for teenagers when they know they're all being held to a common standard by a network of adults who care about their health and safety.
 
• Send a message loud and clear to all teens in your community that drinking and driving is never OK. Let your son and his friends know they can call you to pick them up any time, day or night, if they ever encounter an unsafe situation.
 
• Encourage your local school or church to use peer support groups to help kids learn how to handle the pressure to drink or engage in other risky behavior. Teenagers often can hear a message from their peers that they refuse to hear from their parents. A few years ago I watched a room of spellbound teenagers listen to a 17-year-old describe his experience in a treatment program for alcohol abuse. What I will never forget is his description of the many warning signs his parents did not (and would not) see, and how much he longed to be caught so that someone would set the limits he was unable to set for himself.

Dr. Erickson is a senior fellow and director of the Harris Programs in the Center for Early Childhood Education at the University of Minnesota
 
Want to hear more parenting advice?
Dr. Erickson and her daughter can be heard every Sunday, from 2 - 4 pm, on “Good Enough Moms,” on FM107.1 radio in the Twin Cities or via Webcast at www.FM1071.com