Wikipedia:Reference desk/Archives/Science/2021 September 21

= September 21 =

Bouncing upwards from walls
In various Super Mario Maker levels I have watched on YouTube, Mario can jump seemingly arbitrarily long distances upwards if he's inside a canyon between two walls, simply by hitting each wall in turn and bouncing off it. Surely this can't be possible in real life, even discounting any damage hitting a wall does to Mario's body? J I P &#124; Talk 02:12, 21 September 2021 (UTC)
 * Wall jumping is a well established video game tradition. It appears in many games.
 * But can it be done in real life? Well, obviously nobody can jump four times their own height like Mario does, but could it be done with normal human-scale jumps?
 * A single wall-jump is a pretty common parkour move, but there are a few YouTube videos that purport to show repeated wall-jumps up a narrow gap. (example) Whether these are real or not is another question.  It doesn't seem impossible, but I sure couldn't do it. ApLundell (talk) 03:06, 21 September 2021 (UTC)


 * The laws of physics allow this. How high a human can jump in a simple jump is determined by the kinetic energy that can be reached at the instant of launch, which is limited by human physiology. In the parkour figure additional energy can be input at each bounce (assuming enough friction), so in principle the athlete can jump higher than they could do in a simple high jump. There is again a limit, but one that is more like the limit in how fast a runner can run (see ), where the issue is the delivery of a sustained burst of energy. --Lambiam 10:21, 21 September 2021 (UTC)


 * Physics allows it. Suppose the horizontal component of the velocity of the jumper with mass $$m$$ alternates between $$-v_x$$ and $$v_x$$. At every bounce the jumper exchanges a perpendicular impulse $$J_{\perp}=\pm2v_x m$$ with a wall. While exchanging this impulse with the wall, there is a normal force acting beteen the jumper and the wall, which allows friction to exchange some impulse in a direction parallel to the wall. This parallel impulse has a magnitude of at most $$J_{\parallel,lim}=2v_x m \mu$$, with $$\mu$$ the coefficient of friction. The jumper moves back and forth over a distance of $$d$$, which is slightly less than the separation between the walls as the jumper has finite size. The time between two bounces is $$T=\frac{d}{v_x}$$, giving a limiting average vertical acceleration of $$a_{lim}=\frac{J_{\parallel,lim}}{mT}=\frac{2v_x^2\mu}{d}$$. As long as $$a_{lim}\ge g$$, with $$g$$ the acceleration of gravity, it is possible to climb the wall arbitrarily high (but as exhaustion sets in, your $$v_x$$ will drop). Plugging in some realistic numbers, suppose you can keep your horizontal speed at 3m/s, moving back and forth over 3m (bouncing once per second) and have a friction coefficient of 0.7, you can handle an acceleration of 4.2m/s2. That's enough to do this on the Moon, but not on Earth. I guess a proper athlete could do this on Earth though. PiusImpavidus (talk) 10:56, 21 September 2021 (UTC)


 * Wall jumping by real people can be done with some difficulty. It is done in parkour and freerunning.  See this video for the process one man does to learn the discipline.  -- Jayron 32 15:13, 21 September 2021 (UTC)  Ed: Never mind.  I'm obviously an idiot.  Pay no attention to me.  -- Jayron 32 16:14, 21 September 2021 (UTC)
 * That's the same video I posted. Except that version looks like it's been copied off youtube, posted on Instagram in a different aspect ratio, then copied off that site and re-uploaded to YouTube. (And re-compressed each time.) ApLundell (talk) 15:18, 21 September 2021 (UTC)

Is it necessary to take bullet out off human body?
In movies, doctors struggle to take out bullets in human bodies. But there is a reality case, where the man lived with a bullet in head for years. Source. Is it necessary to take bullet out off human body? Rizosome (talk) 07:14, 21 September 2021 (UTC)
 * Doctors prefer to remove the bullet or bullet fragments, as they could cause infection or other trouble. They may elect to leave one in if the procedure to remove it is more risky than leaving it in. This is commonplace, and sometimes the bullet works itself out of the body on its own, like a splinter. Abductive  (reasoning) 09:32, 21 September 2021 (UTC)


 * (ec) Sometimes it's too dangerous to try to remove them, e.g. They Survived Mass Shootings. Now They Are Living With Bullets Inside Them. (The New York Times), They Survived Mass Shootings. Years Later, The Bullets Are Still Trying to Kill Them (lead poisoning, Time) Clarityfiend (talk) 09:40, 21 September 2021 (UTC)


 * More on lead poisoning. https://pubmed.ncbi.nlm.nih.gov/8057396/ 41.165.67.114 (talk) 12:09, 21 September 2021 (UTC)
 * Did we just assume bullets are made of lead? 67.165.185.178 (talk) 12:56, 21 September 2021 (UTC).
 * They often are. But regardless of what they're made of, they're unlikely to be sterile when they enter the body. ←Baseball Bugs What's up, Doc? carrots→ 13:05, 21 September 2021 (UTC)
 * We believe that in GSW [gunshot wounds] of the spine, retained bullets do not increase the likelihood of septic complications.  Alansplodge (talk) 14:21, 21 September 2021 (UTC)
 * There are no standard medical guidelines regarding bullet removal and the full extent of the consequences of RBF [retained bullet fragments] remains unknown... Meta-analysis demonstrated BLL (blood lead level) significantly higher in individuals with RBF as compared to controls.  Alansplodge (talk) 14:21, 21 September 2021 (UTC)


 * Theodore Roosevelt carried a bullet inside him for the last seven years of his life, after his failed assassination attempt. --  Jack of Oz   [pleasantries]  22:22, 21 September 2021 (UTC)
 * That's a lie! Every person Teddy tried to eradicate ended up dead. Clarityfiend (talk) 10:35, 22 September 2021 (UTC)