On Tue, Dec 02, 2003 at 05:00:20PM -0600, Carl Wilhelm Soderstrom wrote: > On 12/02 04:29 , Matt Murphy wrote: > > It could be stealthy, at first glance it's one of those goofy aftermarket bumpers, but hit the right switch and the SUV in front of you goes a rollin' over. > > I always thought it would be a good idea to have heavy-duty spring-loaded > bumpers on cars, just like you see on older European railroad cars. I > suspect the reason cars aren't generally fitted this way, is because: > > 1. it would add to the cost, and cheaper is usually considered to be better > 2. the car companies make an outrageous fortune by selling you replacement > parts that cost a lot of money, and only fit one particular model year and > vehicle. They love crumple zones; it means they can sell you a good chunk > of a new car! Actually, spring loaded bumpers would make cars (especially small cars) considerably more dangerous. This may be counter-intuitive, but crumple zones really are your friend. WARNING: I am going go off on another physics rant. If you are not into this kind of stuff stop reading immediately, or your eyes might glaze over and steam might come out of your ears (or worse). Car collisions are dangerous to passengers for several reasons. The first danger time is right at the instant of collision. In a collision the total momentum (mass times velocity) is conserved. This means that sum of the initial momentum of the cars is equal to the sum of the final momentum of the two cars : m1 v1i + m2 v2i = m1 v1f + m2 v2f . The danger for the passenger is from this quick change in velocity. The change in velocity leads to forces on the passenger (which are provided by the seat, seat belts, air bag, dashboard, etc.) that are equal to the change in momentum divided by the time it takes for the collision to occur. Its in this time for the collision to occur that the crumple zone shows the first part of its value. By crumpling, the car increases the time the collision takes, which decreases the size of the forces exerted on the person. So by crumpling, the car decreases the damage done to passengers by the original collision. The next place where crumple zones help you, and spring-loaded bumpers would kill you is with kinetic energy. In collision kinetic energy (one half the mass times the speed squared) may or may not be conserved. If kinetic energy is conserved the collision is called elastic, and if not it is called inelastic. In an elastic collision the kinetic energy that is lost is changed into other forms (heat, light, sound, etc.). The most possible kinetic energy is lost if two objects stick to each other after a collision - this is called a totally inelastic collision. An example of an elastic collision is when two billiard balls collide, while two balls of clay colliding and sticking together would be a totally inelastic collision. So how does this relate to car collisions? Well, I would argue that you want car collisions to be as inelastic as possible. In an inelastic collision much of the kinetic energy is absorbed by the bodies of the cars. This leaves the cars with less speed for an secondary collisions that might occur after the first collision, and also makes it less likely that the cars might flip over. This is where crumple zones on cars really shine. The crumple zone will absorb a huge amount of kinetic energy. On the flip side, this is also why spring-loaded bumpers are an awful idea. In a perfectly elastic collision with a small car hitting a larger car, the small car would often move off *faster* after the collision. (I won't bore you with the math on that here, but its like the old trick of dropping a tennis ball on top of a basketball. After the basketball hits the ground, the tennis ball goes flying off extremely fast.) Having the small car fly off faster will increase damage done to the passenger by the original collision, and will also make secondary collisions much more dangerous and increase the chance of flipping over. Anyway, the point is while crumple zones do make it more likely that there will be an expensive body-shop bill waiting for you after a collision, they also make it much more likely that you you will walk away from the collision. While I am at it let me put my two cents in on the big car / little car collision problem. The big advantage for the passengers in the big cars are goes back to momentum. In a totally inelastic collision, the final velocity of the two cars stuck together will be closer to the initial velocity of the larger car, which means that the passengers of the larger car will have smaller forces applied to them at the initial collision. (This is actually a problem that my students had to do a month or so ago.) Of course this ignores the roll-over risks. -- Jim Crumley |Twin Cities Linux Users Group Mailing List (TCLUG) crumley at fields.space.umn.edu |Minneapolis/St. Paul, Minnesota Ruthless Debian Zealot |http://www.mn-linux.org/ Never laugh at live dragons |Dmitry's free,Jon's next? http://faircopyright.org _______________________________________________ TCLUG Mailing List - Minneapolis/St. Paul, Minnesota http://www.mn-linux.org tclug-list at mn-linux.org https://mailman.real-time.com/mailman/listinfo/tclug-list