Re:Every single Onewheel is being recalled after four deaths
by perihelions on Sunday October 01, 2023 @06:23AM
(#37724516)
Falling from a standing position is falling with zero initial angular momentum. A stopped board will pull your feet backwards by friction, which torques your entire body.
Estimate for the worst-case: suppose your feet instantaneously decelerate from 10 m/s to 0 (like from an impact with an immovable object, a pavement gap); suppose they're 1 meter from your center of mass; and suppose you're effectively a rigid solid body over this timescale. Then, you are now rotating 10 radians/second or 10/(2π) revolutions per second. To rotate forward 90 degrees (to the faceplant) takes (π/2)/10 ~ 0.157 seconds.
In contrast: 157 milliseconds of free fall, starting from zero, would only go 12 centimeters.
If you slow things down enough to bring human reflexes into the picture, than the assumptions behind this picture break down. No more "effective rigid solid body". Landing on your feet for example: recover into a sprint (autonomous reflex—faster than nerve RTT between legs and brain), and you are no longer a rotating solid—you are now multiple rotating solids whose angular momenta cancel each other out. You recover vertically, or fall much more slowly.
Estimate for the worst-case: suppose your feet instantaneously decelerate from 10 m/s to 0 (like from an impact with an immovable object, a pavement gap); suppose they're 1 meter from your center of mass; and suppose you're effectively a rigid solid body over this timescale. Then, you are now rotating 10 radians/second or 10/(2π) revolutions per second. To rotate forward 90 degrees (to the faceplant) takes (π/2)/10 ~ 0.157 seconds.
In contrast: 157 milliseconds of free fall, starting from zero, would only go 12 centimeters.
If you slow things down enough to bring human reflexes into the picture, than the assumptions behind this picture break down. No more "effective rigid solid body". Landing on your feet for example: recover into a sprint (autonomous reflex—faster than nerve RTT between legs and brain), and you are no longer a rotating solid—you are now multiple rotating solids whose angular momenta cancel each other out. You recover vertically, or fall much more slowly.