Physics Class

Justin over at ski-blog.com came across an interesting little article on statistics for snowsport injuries. The interesting bit to me was this line:

Helmets are designed to protect your head up to 12 mph, however, most collisions with trees involve the skier/boarder traveling at least twice to three times that speed.

For some reason the 12 mph really stuck out in my head. First I'd like to know where that number originated from, as I've been unable to find any stats to back the number (and the cited data source doesn't seem to mention it) . But let's assume it's true for the sake of this post and correct me where I may have gone wrong in calculations and/or reasoning. Following the Coast Guard CHRIS Manual's conversion factors 12 MPH * 0.4470 = 5.364 meters per second. Assuming no other factors and our head drops straight down from a 1.80 m height (look 180 cm skis), we'll be moving at a speed of 5.939 m/s. [Here's the math: 1/2 * m * v^2 == m * g * h ==> sqrt(2 * g * h) = v. G = 9.8 m/s/s and h = 1.80 m]

That number seems to exceed the maximum impact absorption rate of the helmet. The International Society for Skiing Safety lists a Snell standard for a flat anvil hit on a helmet as 14.1 mph which puts the impact rating at 6.3027 m/s and safely in a flat fall impact range (unless you stand over 2 meters in height).

I'm not sure where this post is going, but I'm putting it online even in it's unfinished state. Hopefully some readers will have feedback on how far off I am.