Shoutout for practical math in a post

December 6, 2005 1:06 PM

eriko does some fantastic pratical math in this post, describing why 150MPH isn't such a good idea on your motorbike.
posted by Ogre Lawless to MetaFilter-Related at 1:06 PM (45 comments total)

Hear, hear. Already flagged as fantastic. When it comes to it, eriko has a track record of brilliant, insightful and deep comments on an almost ludicrously broad range of topics. The fellow is a credit to MeFi.

/not a sock puppet
posted by Faint of Butt at 1:10 PM on December 6, 2005

[!]
posted by ori at 1:10 PM on December 6, 2005

interestingly enough, before I read that (certainly awesome) comment, I was under the impression that 150MPH on a motorbike was in fact a good idea.
whew!
posted by matteo at 1:59 PM on December 6, 2005

I do math every once in a while on this site and no one every complements me.
posted by delmoi at 2:13 PM on December 6, 2005

I also flagged that post as fantastic. It was a very nice job of demonstrating the factors, forces, and distances involved in high-speed driving.

I do math every once in a while on this site and no one every complements me.

everyone is special delmoi but eriko is more special than you today sorry :(
posted by Optimus Chyme at 2:17 PM on December 6, 2005

Oh good, a thread where we can compliment eriko. The best thing about his math-heavy comments are their cogency; you needn't be particularly brilliant yourself to understand him because he's so careful to explain each step of his thinking in clear language. He'd make a wonderful teacher.
posted by melissa may at 2:29 PM on December 6, 2005

Er, this is just distance = rate * time, right? Meh. I once proved the seaworthiness of a typical bathtub.

Nothing against eriko, of course. I guess the former physics major in me is just depressed to realize that not everyone habitually makes those sorts of calculations when confronted by collision scenarios. I guess this also explains why so many local SUV/pickup drivers fail to decrease their speed when the roads get icy.
posted by Galvatron at 2:30 PM on December 6, 2005

Actually, Galvatron, I just think omg, thats pretty crazy, id hate to be one of those peeps. If you and I ever had a long conversation you'd need a course of SSRIs to recover.
posted by melissa may at 2:34 PM on December 6, 2005

His math may be good, and his heart in the right place, but his reasoning is spurious. I'd be too polite to point out such irrelevant things in the thread there, but here it seems appropriate.

1: The drag race start sequence is entirely irrelevent to the question of human reaction time. It does coincidentally give something reasonably close to the average human reaction time for someone paying attention, but in practice there's some pretty wide variation. I seem to recall reaction time studies that put the minimum closer to 0.2 seconds, and the average close to 0.4.

2: Something instantly materializing on the road 88 feet in front of you is not really the most likely way to die. If visibility is so poor that you can only see that far, or if you're worried about things so small that you can't see them from greater distance (which you might well be at 150mph on a bike), or if there are obstructions from behind which something could jump directly onto the road with no warning, then the analysis applies. In any type of circumstance where anybody would even suspect the possibility that it might be possible to go that fast without killing something, it doesn't.
posted by sfenders at 3:12 PM on December 6, 2005

I appreciated the post as well, but had a strange reaction to the feet per second bit. Somehow, this made me want to see how fast I could get a treadmill to go and then jump on and see what happened. It's the closest I'll get 155 mph. Ah math, you do wonders for me.
posted by allen.spaulding at 3:12 PM on December 6, 2005

Er, this is just distance = rate * time, right? Meh. I once proved the seaworthiness of a typical bathtub.

Yeah... It's simple arithmetic with a few unit conversions. Calculating distance based on the time, and uh, the distance/time is not exactly that impressive.

In this commented i estimated the amount of methane in the earths atmosphere based on only the ratio of methane in the air and standard pressure at sealevel.
posted by delmoi at 3:15 PM on December 6, 2005

1: The drag race start sequence is entirely irrelevent to the question of human reaction time. It does coincidentally give something reasonably close to the average human reaction time for someone paying attention, but in practice there's some pretty wide variation. I seem to recall reaction time studies that put the minimum closer to 0.2 seconds, and the average close to 0.4.

That's true, but I think the reasoning was that it would take at least four tenths of a second to engage the breaks, but doesn't a drag racer need to do a lot more then just squeeze a hand-break?

On the other hand, I was under the impression that human reaction time was at lest about .23 seconds minimum, so 0.4 is a reasonable estimate, but not because of drag racing...
posted by delmoi at 3:19 PM on December 6, 2005

Yes, guys, you are good at math, we get it. The MeTa threads starring you will one day come and then eriko can bitch about how one day he calculated the precise number of spermatazoa in God's left nut and we can start the whole bitchfest over again.
posted by Optimus Chyme at 3:19 PM on December 6, 2005

he calculated the precise number of spermatazoa in God's left nut

Now, see, that would be worth a Metatalk post.
posted by Galvatron at 3:25 PM on December 6, 2005

Yeah, well I discovered a truly remarkable proof that xⁿ + yⁿ = zⁿ has no non-zero integer solutions for x, y, and z when n > 2, but this comment box is too small to contain it.
posted by languagehat at 3:32 PM on December 6, 2005

Yeah, well I discovered a truly remarkable proof that xn + yn = zn has no non-zero integer solutions for x, y, and z when n > 2, but this comment box is too small to contain it.

Just as well. Its probably a little too technical fer mat and the rest of us.
posted by gsteff at 3:53 PM on December 6, 2005

gsteff scores!
posted by Wolfdog at 3:59 PM on December 6, 2005

Seriously people, the guy wrote an interesting, easy to read comment and all some of you do is whine because no one complimented you on something similar? Get over yourselves. Give the guy a pat on the back and play nice.
posted by chiababe at 3:59 PM on December 6, 2005

*lights the Fermatsignal, pages James Harris*
posted by cortex at 4:00 PM on December 6, 2005

eriko's explanation was cool. Now I want to know how much damage I can do at a walking pace (5'11" 200lbs), and whether anyone can react fast enough to avoid the devestation.
posted by carsonb at 4:03 PM on December 6, 2005

"I do math every once in a while on this site and no one every complements me."

Yeah, dumb math, with your butt.

"Yeah, well I discovered a truly remarkable proof that xn + yn = zn has no non-zero integer solutions for x, y, and z when n > 2, but this comment box is too small to contain it."

Hmm. I could swear I saw something like this on MeCha, only funnier. Hmm...
posted by klangklangston at 4:06 PM on December 6, 2005

1+1=2?
posted by seanyboy at 4:41 PM on December 6, 2005

and a very fine back of the napkin to you all!
posted by blue_beetle at 5:04 PM on December 6, 2005

In Olympic track events (and others, I suppose), 1/10th of a second is considered the absolute minimum reaction time, so that anyone who leaves the blocks less than 0.10 seconds after the starting gun is fired is considered a false start. Average reaction times are 0.15 seconds. These are for world class athletes completely focused on one thing: getting off the blocks as quickly as possible, so 0.40 seconds sounds like a perfectly reasonable estimate of reaction time for an idiot on a motorcycle.
posted by zanni at 5:20 PM on December 6, 2005

zanni: Don't those people know when they're going to start? The reason human reaction time is about two tenths of a second is that that's how long it takes for the electrical signal to reach your hands from your brain. If you can anticipate when you're going to start you can 'react' much more quickly.
posted by delmoi at 5:33 PM on December 6, 2005

What I was trying to get at is that the drag racing start sequence isn't about reacting as quickly as possible. It's about reacting in exactly the right amount of time.
posted by sfenders at 5:42 PM on December 6, 2005

I once proved that carrying change is a worthwhile activity. Sadly, it was not on MeFi.
posted by Plutor at 5:43 PM on December 6, 2005

delmoi writes "That's true, but I think the reasoning was that it would take at least four tenths of a second to engage the breaks, but doesn't a drag racer need to do a lot more then just squeeze a hand-break? "

Nope: Mash the loud pedal while releasing the brake and then hang on.
posted by Mitheral at 5:55 PM on December 6, 2005

Plutor: I don't think it's a question of the energy required to carry the change, but pocket-space. Also, calorie value changes with how much you've already eaten, and for a lot of people eating more then a certain amount of calories has a negative utility.

A more interesting approach would be to figure out how likely it is that you're going to just end up getting more change if you dispose of the change you already have. Like, if you've got 55¢ will you be less likely to spend more time carrying around more coins?

Oh well.

this along with this were probably my favorite math posts... although I should have written dx(t) and dy(t) rather then x(t) and y(t). Oh well. the calculator syntax was correct.
posted by delmoi at 6:04 PM on December 6, 2005

I right "Oh Well" way to much.
posted by delmoi at 6:06 PM on December 6, 2005

Far out. I flagged it too. At about comment number 77, before I saw this thread.
posted by uncanny hengeman at 7:34 PM on December 6, 2005

I do math every once in a while on this site and no one every complements me.

But when it comes to grammar, delmoi, virtually everyone here complements you.
posted by weapons-grade pandemonium at 8:46 PM on December 6, 2005

Not sure if this means anything but the reaction times in drag racing take into account the time it takes to actually get the car to move, too. That is, the timer starts when the light goes green, and the RT is calculated when the front tire of the car actually moves. So there's more than just nervous system stuff, there's the engaging of the clutch and throttle, plus the entire drivetrain beginning to move. The driver's reaction time is probably a bit less than 0.4s.
posted by knave at 9:01 PM on December 6, 2005

If anybody's handing out a math award, it ain't me. For me, eriko's calculations made the motorcycle-vs-car-at-150MPH a lot more approachable. How'd such a wreck happen? Hell, glancing over your shoulder and back would chew up 200 ft right there. The guy would appear as if out of hyperspace.

I found eriko's comments good in the sort of way I haven't seen much of since I first became enamored with the old Blue -- to the topic, involving a little elbow work and greatly furthering the breadth of the original post.
posted by Ogre Lawless at 9:45 PM on December 6, 2005

The drag race start sequence is entirely irrelevent to the question of human reaction time. It does coincidentally give something reasonably close to the average human reaction time for someone paying attention, but in practice there's some pretty wide variation. I seem to recall reaction time studies that put the minimum closer to 0.2 seconds, and the average close to 0.4.

What sfenders said.

Crap post, eriko.

;)
posted by uncanny hengeman at 12:25 AM on December 7, 2005

HyperBlue's post is the best and most important of that thread, actually.
posted by majick at 12:32 AM on December 7, 2005

Math is power!
posted by TwelveTwo at 1:27 AM on December 7, 2005

This is one of the densest real-nerd-jokes-per-comment threads I've seen in a long time. Props to languagehat and weapons-grade pandemonium.
posted by Plutor at 4:08 AM on December 7, 2005

Pay attention to me!!!
posted by chunking express at 5:08 AM on December 7, 2005

Gawsh, my own call out. I'm blushing.

As to reaction time. Yeah, it's a handwave. I think it's a defensible handwave, with no more than an order of magnitude error. There are ways that you can reduce it, but the error is more likely to go the other way. Vibration means that your visual acuity will be lower, and if you happen to be looking at another potential problem, you might lose a second (and 200 feet) before you even see the problem. Add in the fact that a drag racer is primed to do a specific thing, and you have to figure out what you need to do first means that .4 seconds is, I think, on the low side.

But, being low, it served to prove the point -- that you can eat a fantastic amount of space in very little time.

As to the math. Delmoi's got a point, in that the math, as such, is trivial -- just conversion, then a division for distance. The art of the lesson, though, is getting things understandable.

"155mph" isn't very useful, and "13,200 feet in a minute" is even less so. But lots of people have flown, and seen the full length of a runway. "One runway" is something they can grasp. "30 paces" is another -- and nobody's arguing my assumption that one pace=3.00 feet. ;)

So, yeah, the numbers are fuzzy -- and in the end, I'm willing to bet that the distance before reaction is too short. Indeed, between the wind roar and the vibration, he probably had two seconds before impact -- and never really grasped that yes, that's a car, right there, in front of me, I'm going to die.

Heck, at 155mph, holding a helmeted head up to take a good look isn't a trivial feat.
posted by eriko at 5:27 AM on December 7, 2005

I right "Oh Well" way to much.

right. Jesus. And I posted that at 6pm last night, so I can't even claim I was tired. *sigh*.
posted by delmoi at 6:25 AM on December 7, 2005

Don't you mean "Oh well"?
posted by Plutor at 6:31 AM on December 7, 2005

Huzzah for numeracy.
posted by gleuschk at 7:40 AM on December 7, 2005

No blood for "Oh well"!
posted by dwordle at 11:46 AM on December 7, 2005

In this commented i estimated the amount of methane in the earths atmosphere based on only the ratio of methane in the air and standard pressure at sealevel.

And you seem to be off by about a factor of 2 or so. The 0.0000017 of the atmosphere which is methane is the fraction by volume, not by weight. The volume of a gas--as long as conditions are reasonably close to ideal, which the atmosphere is--is proportional to the number of moles of that gas, not its mass. To calculate the percentage of the atmosphere which is methane by mass, you'd have to apply a correction of the molecular weight of methane divided by the average molecular weight of molecules in the atmosphere--about 16/29. So we're talking only about 140 million tons of methane, not 250 million tons.
posted by DevilsAdvocate at 3:15 PM on December 7, 2005