Tuesday, October 2, 2007

PopSci's Extreme Sports Error

I love the magazine Popular Science, but nobody's perfect, and this time PopSci slipped up.

In an otherwise excellent article by John Mahoney about physics in the movies, he criticizes the Vin Diesel adventure xXx by saying that there's no way anyone can outrace an avalanche on a snowboard.

Sorry John, you got that wrong. There's a good chance you wouldn't survive, and it's unwise to try, but it's at least theoretically possible to snowboard to safety in front of an avalanche.

The problem is that Mahoney is under the mistaken impression that the downhill speed record on a snowboard is only a piddling 50 miles per hour. In fact, the record is closer to 125 miles per hour (201 kilometers per hour). Even that's a bit slow for my comfort, considering that avalanches typically move (according to Mahoney) at minimum of 130 miles per hour. But with a slight head start, you could certainly stay out of trouble for a little while.

Considering that a slow avalanche would overtake a world class speed snowboarder (which I am assuming Vin's character must have been) at a relative speed of about 5 miles per hour, the gradual approach of the avalanche on Vin's heels would have been a very dramatic (and entirely possible) moment.

Check out a these videos of legendary snowboarder Big Mountain Jeremy Jones racing some avalanches (Jeremy actually performed Vin Diesel's snowboarding stunts in xXx). It seems like a really stupid thing to do. Thank goodness he survived. But as you can see, he snowboards (and falls) at about the same rate as the avalanche.

Mahoney should probably have realized his error by considering the fact that both snowboarders and avalanches descend the mountain with very little friction. In other words, the only force affecting the descent is gravity (to a pretty good approximation anyway).

Galileo showed that when friction and drag are low, all things accelerate (and slide down hills) at the same rate under the pull of gravity, regardless of size or mass. So whether you're a snowboarder, a boulder, or an avalanche, your fastest trip down a mountain is going to be about the same.

Why do some avalanches fall at speeds that top out around 200 mph (as Mahoney notes)? Because they are sliding down VERY steep slopes. If you were to snowboard down those sorts of slopes, you could theoretically go just as fast as the avalanche. Snowboard speed records are generally set on slopes that descend at a little over 30 degrees. If the slopes were steeper, the speed records would be higher.