The Physics of a Firefighter's Window Catch | Hacker News Reader

The Physics of a Firefighter's Window Catch

wired.com

54 pointsalex_young18 comments

What is ignored in the article is that a large part of the momentum was absorbed by the wall the victim must be assumed to have crashed against. Since the firefighter was anchored to the window sill, it was enough for him to hold on to the victim to turn the momentum towards the wall.

Still the firefighter needed a lot of force to hold on to the victim through the tight turn. If his grip hadn't been good, he couldn't have done that. That is very impressive.

Edit: By the looks of it the victim was held at the legs. We can expect a bad concussion.

Agreed, grip strength and reaction speed were the most impressive parts of this feat. His teammate anchoring him deserves about half the credit for non grip related strength.

"Turning momentum against the wall" needs actually more force than just stopping the faller. If the faller initially falls with momentum -p z (where I will denote the unit vectors of the coordinate system with x, z, and p is the magnitude of the momentum) and turns that into p x, then you need an average force of Favg=deltap /t= sqrt(2)p /t, while just stopping would only be Favg=p/t, where deltap is the total change in momentum and t is the duration of the stopping.

The reason for that is, that momentum is a vector and so you need to supply force in along each of the coordinate axis individually.

You're assuming a 90 degree turn. It doesn't take a 90 degree turn, however--they were probably still heading mostly down when they struck the wall. The wall (and the falling person) would absorb most of the impact. Not good for the faller but far better than keeping on going.

> By the looks of it the victim was held at the legs. We can expect a bad concussion.

If the victim had hit the ground, we can anticipate worse.

The link below has video with audio of the victim crashing against the wall during the catch, and it certainly sounds like he got a pretty bad knock.

https://www.thesun.co.uk/news/6426086/incredible-moment-fire...

Just have a look at the comments of the video on YouTube. That sums up why I no longer visit Reddit, and turn off comments on most sites. Impressive catch though!

https://www.youtube.com/watch?v=eqHloZxSeGo

I think that should be why you don't read youtube comments, and have nothing to do with reddit.

Look at the comments on the post from reddit instead: https://www.reddit.com/r/videos/comments/8nk31a/latvian_fire...

Yes, but there have been a few gems (not sarcasm) that I find sifting through all those comments that I've found very useful. It's like digging through shit to find something valuable.

Just what I want to waste my time doing.

Another way to look at this: it takes about as much work to stop someone falling from 3m as it does to launch them 3m in the air. So if you can stop the guy falling, you could instead throw him into the air to the same height. It's not quite that simple (different muscle groups, etc) but a good approximation.

Flinging 50kg to a height of 3m above the release point seems a little high. I'm a fit adult male, although neither an athlete nor a fireman, and I can manage about 15cm. So I think friction is doing a significant part of the work here, likely between the fireman's clothes and the wall or window sill.

Still an impressive catch of course!

Or carrying them up a flight of stairs on your back. I think peak power is more important than work here. The peak power to throw someone 3 metres into the air is ludicrous, but arresting a fall over a (slightly) longer time period, with bones and ligaments taking the peak power instead of muscles are what is going on.

> with bones and ligaments taking the peak power instead of muscles

That's kind of an odd way of phrasing it; I would have said something along the lines of them providing the upwards force through tension rather than bringing power into the discussion.

It's the time derivative of work, seemed natural to me. Also muscles use rate limited chemical reactions to move, so it is a reasonable assumption to model them as power limited (with different regimes for different metabolic mechanisms). So when talking about the limits of what a person can do with their muscles, power limits seem important.

Your comparison of throwing and catching makes me think of the Caber Toss. Where a 79kg wood trunk gets, well, tossed.

https://en.wikipedia.org/wiki/Caber_toss

The key to this, and to mentioned in the article, is accelerating/decelerating not the object, but the centre of gravity.

You can dissipate a lot more energy than you can produce with your muscles. For example, you can comfortably drop much more than you can jump.

Likewise impressed, but the fireman's own body transmitted much of the force to the window frame. So it's less the equivalent of launching someone into the air, and more like hanging on to someone while you are launched into the air by harness.

I thought the article would be about how you get a grip that don't slip when the person flies past you.

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