Science desk
< August 4	<< Jul \| August \| Sep >>	August 6 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 5[edit]

How can this puncture a tire?[edit]

Wouldn't the tire just press flat the puncturing device, instead of the puncturing device puncturing flat the tire? Unless the picture below is not accurate, I fail to see how this could work. If the rubber piece had 3-4 nails attached, it would be a different scenario though.

— Preceding unsigned comment added by Llaanngg (talk • contribs)

The device appears to be an improvised Caltrop. The large rubber disc attached to it would cause the nail to "flip up" as it is run over, driving the nail into the tire. As noted in the article Improvised weapon, the device aren't used singly, but rather a batch of them is thrown across a road; one may be easy to avoid; but if your car hits 100, even if only 10% work as intended you've shredded your tires. --Jayron 32 18:30, 5 August 2016 (UTC)[reply]

Mmmm, I also don't get why would a nail with a rubber disc flip up and punch a tire when you drive over it . Notice that in the case of a caltrop, there is not only a spine pointing up, but it also has a stable base. Why would it also work with a flexible base?

Equally, I also wonder why they didn't use more nails to make it look more like a caltrop. If you got three nails in a triangular layout, the efficacy would be higher. That could make sense even if nails are scarce goods in Palestine (where these were used). --Hofhof (talk) 20:59, 5 August 2016 (UTC)[reply]

The traditional three-nail design requires welding, which means (fairly) expensive equipment and a (moderate) level of technical skill. This design just requires a hammer, a nail, and a piece of rubber. Tevildo (talk) 11:54, 6 August 2016 (UTC)[reply]

I mean, they could have used three nails nailed to a piece of rubber. Indeed, other alternative caltrops are a rubber tube with nails pointing in all directions. Hofhof (talk) 12:03, 6 August 2016 (UTC)[reply]

Looks like a washer-head nail to me, not an improvised weapon. They're usually used to nail sheets of something that won't hold nails well to a substrate, or which will leak if not sealed. The ones with rubber washers are usually for roofing applications where they'll remain exposed. That said, they're effective tire-puncturers, they can flip up when hit, and while 9 out of 10 might not hit right, the 10th is a real nuisance. Decking screws are common hazards too, despite the smaller head. The traditional malicious tire-killer is the common broad-headed roofing nail, which is short enough to stand upright. I make a point of picking up fasteners when I see them at parking lots and construction sites. As someone who spends a lot of time on construction sites, I average about one tire puncture a year. Acroterion (talk) 12:11, 6 August 2016 (UTC)[reply]

Don't washer head nails, even those with rubber washes tend to have big metal heads too? Nil Einne (talk) 04:33, 7 August 2016 (UTC)[reply]

Also if the rubber "washer" weighs as much or more than the nail, then a decent proportion of the nails will probably land standing up, like a Roly-poly toy. Again if you drop a few dozen, you only need half to stand up, using one nail and one washer might be more economical than using 3 nails per washer. In fact to make a pyramid with more than average chance of standing with a nail upright you probably need 5 or 6 nails per caltrop, so if your "one nail and washer" has a better than 30% chance of standing up it will be more worth your time just making a bunch of these than using 5 or 6 nails that have a 100% chance of standing up. And that's even assuming the picture is a "optimal design" which is a big assumption, the picture could very well be an improvised sub-optimal design, it would still be an improvised weapon intended to puncture tires, do we even want an image of the most optimally designed caltrop design featured on our page as a reference? Vespine (talk) 01:05, 8 August 2016 (UTC)[reply]

Speaking from unfortunate personal experience, the tires most likely to be punctured by a nail are the back ones. The first tire runs over the nail, it recoils from the downward pressure or is spun upwards from the friction, flips upright, and then enters right into the back tire. I can see that happening here also.Timtempleton (talk) 20:51, 8 August 2016 (UTC)[reply]

As Larry Niven put it in Ringworld, "we have brought no weapons, only tools..." It is easy to understand how someone walking near an Olympic bike race (or even run...) would want to have a box of off-the-shelf fasteners, not caltrops. I cannot imagine how Brazil has managed to avoid serious incidents so far... Wnt (talk) 23:12, 9 August 2016 (UTC)[reply]

Sorting M&M[edit]

If I fill a jar with M&M and shake it violently for t time, could I sort them all by colour? That is, get 1 layer for each colour. The colours are brown, orange, red, green, yellow and blue. How long would the time t be?Hofhof (talk) 21:41, 5 August 2016 (UTC)[reply]

In theory, yes. I tend to think you'd get pulverised M&Ms before you got them colour-sorted. --Tagishsimon (talk) 22:06, 5 August 2016 (UTC)[reply]

Many years ago, I visited the research lab of a theoretical physicist who studied exactly this type of problem... I think it was the Granular Materials research lab at University of California, Davis.

Shaking around a bunch of discrete particles, whose shapes are imperfectly spherical, yields a lot of interesting collision-physics and ensemble-physics. Here's one paper that's available for free on arXiv: Segregation and Stability of Binary Granular Mixtures, (2007), about sorting particles by vigorously shaking the mixture.

You've upped the ante by including multiple different colors instead of just two particle types - sounds like perfect material for a follow-up paper!

Nimur (talk) 22:26, 5 August 2016 (UTC)[reply]

Bogosort may be relevant. -- BenRG (talk) 00:39, 7 August 2016 (UTC)[reply]

It's theoretically possible but entirely improbable. A similar question: say you have a box with a vacuum on one side, a gas on the other, and a barrier in between. When you remove the barrier, the gas fills the box. Will the gas ever move completely over to the other side of the box again? --jpgordon^{𝄢𝄆 𝄐𝄇} 15:57, 7 August 2016 (UTC)[reply]

Given enough time (an unimaginably long but still finite period) or a reasonable length of time and a small enough box (just enough to hold, say, a half dozen gas molecules), the answer is a definite yes.

The M&Ms will tend to sort if they last long enough, but I doubt that they will end up sorted by color. Size is more likely. --Guy Macon (talk) 11:01, 9 August 2016 (UTC)[reply]

Brazil nut effect. DMacks (talk) 13:06, 9 August 2016 (UTC)[reply]