Wikipedia:Reference desk/Archives/Science/2015 August 21

= August 21 =

Using newspaper to prevent weeds
Does this hack actually work? Wouldn't the weeds be able to penetrate the newspaper, especially when the latter is wet? Or is the newspaper supposed to prevent the germination of the weed seeds in the first place? My other car is a cadr (talk) 02:59, 21 August 2015 (UTC)


 * just plain old mulch does a similar thing, it's not 100% effective, but reduces weeds considerably. I think it's a combination of blocking out light and providing somewhat of a physical barrier. It's not like young shoots are very strong and if they grow on any angle at all, without sunlight to follow, growing along below the newspaper would take less resistance than trying to punch through it, even if it was like soggy tissue paper. Vespine (talk) 03:51, 21 August 2015 (UTC)


 * Yes, basically the newspaper acts as a mulch. There's nothing particularly special about newspaper, other than it being convenient to apply and a way of recycling/reusing material that would otherwise go to the landfill. Short Brigade Harvester Boris (talk) 03:59, 21 August 2015 (UTC)


 * One more for "yes this works, but not 100%" - 18 months to degrade is of course highly variable, based on moisture and heat. You have to especially watch out for weeds that spread through rhizomes or stolons, such as many grasses. Some seeds are photosenstitive as described at Germination, but many are not and will germinate based on moisture, temperature, and many other cues. You can indeed expect some weeds push through both newspaper and mulch. My WP:OR is that newspaper (or any other scrap paper) under mulch is notably more effective than mulch alone. SemanticMantis (talk) 14:00, 21 August 2015 (UTC)


 * Mulch is going to let bits of light through, unless very thick, while a relatively thin layer of newspaper should be able to block all light. I attribute virtually all of the effectiveness to this.  I agree that newspapers won't physically block a plant from growing through.  Plants can grow through and crack cement, after all.  StuRat (talk) 14:09, 21 August 2015 (UTC)

Why is it so hard to start a tear in plastic film?
Once you have a tear started, it is easy to continue tearing. 50.43.33.62 (talk) 18:42, 21 August 2015 (UTC)
 * I think it is derived from their long-chain molecular structure. Most plastics used in films will tend to deform when placed under tension, spreading the stress over a wider area. Once a tear is started, the stress becomes localised, and will exceed the limit quite easily. I'll see if I can find some sources though - there are sure to be plenty. AndyTheGrump (talk) 19:10, 21 August 2015 (UTC)
 * Yes, that is essentially correct. I've supplied many refs below but I have yet to find a good introductory ref for this very common question! SemanticMantis (talk) 19:30, 21 August 2015 (UTC)


 * (EC)It's sort of the same reason that it's easier to pull apart a zipper once you've broken through "retainer box" and "insertion pin" (i.e. the part at the bottom where you feed the two halves of the zipper together). Basically, at the onset, the force you apply is distributed throughout a line or plane. Once the tear is initiated, all of that force gets sort of "focused" directly into the weak spot. Tearing is basically an example of fracture mechanics, and the details can get very complicated!
 * Unfortunately we don't have good simple coverage of tearing on WP - both tear initiation and tear propagation are just redirects to tear resistance. Here's a decent and more accessible overview - that is where I recommend you start. Note that the ductile case of plastics works a bit differently than the brittle case.
 * Here's a book that also has some good info . For any given material, it can be very complicated to predict the properties of tearing, so there are lots of empirical tests, e.g. those described here . Related physics concepts are tensile strength, Stress_(mechanics), Strain_rate Young's_modulus, Elastic_modulus, and Yield (engineering). (I had a lot of this ready because I've looked into this before - if anyone has a better introductory document on tearing I'd love to see it!) SemanticMantis (talk) 19:30, 21 August 2015 (UTC)
 * Our article on fracture mechanics may possibly be relevant. AndyTheGrump (talk)

Part of the reason is already in the name: See Plasticity (physics). Just as when you're tearing on the cheese on your pizza, it just gets thinner and thinner, instead of breaking. &mdash; Sebastian 20:24, 21 August 2015 (UTC)

Thank you. I made a blog post out of this: http://pergelator.blogspot.com/2015/08/rip-tear-crack-split-fracture.html 50.43.33.62 (talk) 17:25, 23 August 2015 (UTC)


 * Hey, that's pretty cool, thank you too! SemanticMantis (talk) 16:06, 24 August 2015 (UTC)


 * I've noticed Polymer banknote are an extreme example of this. It's practically impossible to tear one of these with your hands, but if a tear gets started, it's extremely easy to tear. Vespine (talk) 22:36, 23 August 2015 (UTC)

Why are some plastics easy to start tearing but get tougher as you tear ?
I've noticed the exact opposite in certain plastics. Why the difference ? One example is the plastic wrap around Scott brand toilet paper. I find I need to poke a hole with a finger, tear it a bit, then poke another hole, tear a bit more, etc.  StuRat (talk) 02:04, 22 August 2015 (UTC)


 * Direction of the tear? I suspect the direction matters.  --DHeyward (talk) 02:16, 22 August 2015 (UTC)


 * I think you're talking about early tear propagation compared to late tear propagation, not initiation compared to propagation. But your example does make a lot of sense and rings true for me. If you look at my bold link above, it mentions that some plastics tear to a stable point, wherein the tear will not spread without further increase in force. Compare to e.g. aluminum foil which will tear indefinitely with the same small force. I also think something similar to work hardening might be going on - the boundary layer of the tear sort of gets built up with lots of stretched material concentrated into a small space, and the chains become aligned in a more circular fashion compared to the original film. SemanticMantis (talk) 13:35, 22 August 2015 (UTC)