If we want to be able to deal with weak, elderly, or injured people, and we also can expect them to not in the perfect posture in the net, then I’d say 5G is utterly safe. Somebody flat on their back in good condition can take a lot more, but 5G is on the order of what NASA uses for rocket takeoffs for a reason.
Assuming we’re trying to slow somebody down from a decent terminal velocity guestimate of 150fs-1 (thanks @md-2000) and we’re limited to 5Gs = 155 fs-2 that says the decel from there to zero fs-1 will consume 72.5 feet. If we assume a semi-gentle buildup to 5Gs and semi-gentle cessation of 5Gs at the bottom maybe more like 100 ft.
Thank you, that’s awesome! That said, my poor physics education is catching up with me, so forgive me as I continue to ask dumb questions.
I was playing with the idea of not reaching perfect 0 velocity by ground level, but the more I think about it, the more I think it needs to be extremely close to that. Maybe 5-10 feet per second would be okay? I’m imagining a person falling at that speed, and it’s difficult for me to imagine serious injuries, but I could well be wrong. I guess 5 fps is about the average walking speed, so landing at this speed would be roughly equivalent to walking into a glass door: painful, but unless the ground shatters, not necessarily injurious? Is that a reasonable way to look at it?
Handy conversion guide I learned/memorized long ago - 60mph = 88fps. So it’s not hard to then calculate from that any mph in fps; where g=32fpsps so simplifies calculations.
100mph ==> 88x100/60=146.6fps
100mph is often given as approxiamte terminal velocity of humans, but of course that depends on a lot of things, like body angle, what sort of clothing produce sail/parchute effects, person’s weight vs area, etc etc.
I vaguely recall air force(?) testing that people tended to black out in the neighbourhood of 10G. But again, that depends on how long and what angle they are at. The airforce, obviously, is worried about people sitting in pilot seats, especially when ejecting.
If you’re saving someone in a net, blacking out isn’t necessarily a problem. But a person may not be ideally positioned in the net: arms or legs might be twisted into weird positions, head turned to the side, etc. Can lethal ruptures or breaks happen at higher g if bodies are in weird positions?
Yep - here in New Zealand (with lots of rugged bush country and near-shore waters with no mobile coverage, and lots of people who want to explore those areas) one of the mobile service providers is already advertising on providing country-wide (and probably near off-shore) satellite-based text communications.
Personally, I would prefer to take a PLB (personal locator beacon) but if satellite provides suitable coverage I can see it saving lives.
It’ll be amazing when it’s more widespread. The latest iPhones already have an emergency beacon system. It’s limited, and you might have to wait for a satellite to come overhead, but it’s way better than nothing.
AST SpaceMobile put up their Bluewalker satellite, which has an enormous antenna and theoretically allows voice, etc. comms. They’ll need to put up a lot more satellites for full coverage, but it seems their prototype works well. No special phone needed.
Starlink and T-Mobile announced a plan to put cell antenna/transceivers on their next-gen satellites. Also works with any phone, at least those compatible with T-Mobile frequencies. Should handle any kind of traffic. Total bandwidth is limited, but texts and limited voice or even internet should be possible.
There are probably more systems than these coming up; they’re just the ones I knew about off the top of my head. They’ll definitely save lives.
Nitpick: 90.7 feet. You forgot that one of those five Gs is the one from gravity itself.
You will reach 0 velocity at ground level, one way or another. “Hitting the ground at safe speed” just means that you’re using the size of your body an an acceleration distance. But if we’re talking triple-digit heights anyway, you might as well ignore the size of a body.