Physics of Snowboarding (G-forces during jumps) | Hacker News Reader

Physics of Snowboarding (G-forces during jumps)

blog.alpinereplay.com

9 pointsdlokshin4 comments

This is a bit confusing. I'm not sure how much of it is due to them dumbing down the physics for the reader and how much of it is due to them not understanding the physics of what they're doing.

For instance, measuring the acceleration of the snowboarder upon landing. Which part of the snowboarder? If it's measured at the actual board you'll get a very high number. By the time it's transmitted up through the binding, through the padded boot and into the boarder's foot it's less, because all these things have elasticity and they bend, lengthening the shock time and decreasing the instantaneous acceleration. Then it's less and less as it gets transmitted through the foot, knee and hip joints (these being the body's big shock absorbers) as well as the rest of the body (which is somewhat compressible) until, by the time it gets to your head, the actual instantaneous acceleration is much less.

This is why a snowboard jump which ends with you landing on your feet is harmless while a snowboard jump which ends with you landing on your head is very very painful.

We use a phone to measure the acceleration, which in general is either kept in the breast pocket or a pants pocket. In this case it's a pants pocket that is right above the knee.

You're right, your body works as a spring, and it attempts to transmit the full force over as long a period as possible. But you can see from the graph that this takes 60ms. The different in acceleration that you're talking about pretty minimal. Regardless of how good your knees and ankles are at absorbing shock, if you don't clear the tranny (part of the jump where it begins to slope down) you're effed.

More than anything, the point is that they're absorbing a ton of shock.

I'd like to see how the recorded numbers vary depending on the location of the accelerometer. Loose phone in pocket vs phone strapped tightly to the head vs phone strapped tightly to the board. That'd be some interesting science.

With a phone in a pants pocket, are you sure you're not just looking at the spike when the phone, loose in the pocket, hits the edge of the pocket?

The longer-term gee factors, which are what I assume you're really interested in (ie the acceleration during a turn) should still be accurate, but I'm not yet convinced that the shock data is meaningful.

It always annoys me when people throw around numbers like 14g and start comparing themselves to fighter jets. If you jump a few inches into the air and land on a hard surface with your knees locked, the momentary acceleration would be pretty impressive. That doesn't make you an F-15. Fighter jets can maintain those accelerations for seconds on end.

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