Impact

Jake Brown was competing in the X-Games this week and fell 50 feet to the floor after a mishap with this skateboard. He walked away from the fall. You can check out the video at http://www.youtube.com/watch?v=KBvCrSjpx9I

Apparently, the only injuries he has are a minor fracture on a vertebra, a broken bone in his wrist, and some liver damage. Watch the video, that's amazingly light damage for the fall that he took.

At the same time this week, reporters were talking about how a lot of people in the Minneapolis bridge collapse fell 65 feet and not only survived, but walked away uninjured. Many of them went to the hospital 2 or 3 days later, finally complaining about their aches and pains, but still, a 65 foot fall is supposed to be fatal.

So how did these people walk away for falls that are supposed to kill you?

Jake Brown actually had time to think about it and was able to twist around to control his landing to minimize the impact. They interviewed him and he claims he thought about the fall of a friend of his while he was falling. His friend fell from a lesser height than Jake, but broke both legs. So he probably thought it through before he was in trouble. He said in an interview after the accident that he planned to fall the way he did. I heard that Army Airborne parachutists hit with the impact of someone jumping off a 2 story building. The way they are trained to survive the fall is to hit on their feet, crumple by collapsing their legs, and roll out across their side, taking up the rest of the force. I think Jake did somthing similar to this. He hit on his feet and that probably took a great deal of the impact. I still wonder why the legs didn't snap under him.

The people that were in cars on the bridge when it fell claim that there were a series of jolts rather than one long fall. Even though the bridge looks like it drops pretty fast, it wasn't freefall, it was getting hung up on the way down, slowing the fall.

Americans used to drive around without seat belts. Car accidents used to kill around 65,000 people a year in the 60s, and I believe we're down to around 40,000 a year. That's with something like twice the total miles travelled for cars, so I'm guessing we've reduced the fatality rate to 1/4 what it was. We finally decided that enough was enough back then and started developing safety features in cars, starting with the seatbelt.

The way the seatbelt works is by making you part of the car. Before seatbelts, accidents were a series of 2 collisions. Your car hit an object and then you flew forward in the car and hit the windshield or steering column. The second little mini collision between the person and the car is what injures the people inside. When you strap yourself in, you become a part of the car, and it takes the brunt of the impact.

A moving object requires energy to stop. In a collision, that energy is used to deform the car. Think of crushing a soda can with your foot. In an accident the car gets hurt, not the person. When engineers started understanding this, they started designing "crumple zones" into the car. Those are places that the car is designed to smash down safely and not smash the people inside. In most cases, this is the frame in the front, below the engine. They also reinforced the cab so that the area where the people are is less likely to crumple. Two other features they designed were a seat that supported your head and airbags to cushion your forward motion. This impact is supposed to be like being smashed between pillows.

In the case of the Minneapolis bridge collapse, the cars are falling straight down while upright. This is not the way a car is designed to impact, but it is the way a car, and the driver in the car, are designed to be supported. I suspect that many of the cars on the bridge would have flat tires (quite a cushion there, like 4 air bags)and probably snapped axles. That would take a lot of the energy out, and then the people are sitting upright in cushioned seats that would take up the rest.

That's just my theory. This is not something you ever want to really test.