Wikipedia:Reference desk/Archives/Science/2011 October 30

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October 30

botany

i ask question about photo pared that the short day plants cannot give the flower in long day & long day plant cannot give flower in short day. question is that in the gardens whole night the light provide for the garden beauties in whole day also the sun light are available but in spring season the plant in garden give the flower with. There is no effect are accrue with light but in books says that light effect flowerings.(HAROON UR RASEED OF PK.MARDN) — Preceding unsigned comment added by Haroonmuskan (talk • contribs) 04:50, 30 October 2011 (UTC)

If I have understood your question correctly, the answer is that plants are not sensitive to light itself, but to sunlight, which has its own spectrum. If the artificial lights are "full spectrum" lights, then the plants will respond to them. If not, they won't be affected. See Full-spectrum light and Gardening under lights. --TammyMoet (talk) 09:22, 30 October 2011 (UTC)

Note that it's not the full spectrum of lights that is thought to cause the short day/long day effect, but rather specific wavelengths, primarily in the red end of the spectrum [1]. Short day and long day plants have phytochromes which are sensitive to the red end of the spectrum, and they're the one used to regulate photoperiodism. - For the gardeners, the key to enure flowering is to avoid too much red light during the night for short day plants, and to provide sufficiently large amounts of red light during the day for long day plants. Note that the light provided doesn't have to actually look red to work, the red wavelengths in white light work just as well, which is why full-spectrum artificial lights (and sunlight) work. Note also that the reliance on red light is only for photoperiodism - other plant processes, like photosynthesis, need other wavelengths of light as well. -- 174.24.217.108 (talk) 18:11, 30 October 2011 (UTC)

Photon as source of gravity

Do photons have a gravitational field around them ? Suppose a bunch of photons whizz past some uncharged particle like a neutron or anything with mass, will the mass be deflected or attracted to the photon? The photon itself will change course due to the gravitational field of the particle (neglecting electromagnetic, weak and strong interactions). So to conserve the total momentum, I assume, the particle will also move. 117.231.72.65 (talk) 12:41, 30 October 2011 (UTC)

Yes, the photon will have its own gravitational field. The photon has energy, which is equivalent to it having mass (E=mc²), so it has the same kind of gravitational field as anything else with mass. I don't think you could have a one-way gravitational interaction - if something can respond to a gravitational field then it also creates a gravitational field. --Tango (talk) 13:42, 30 October 2011 (UTC)

yes, anything that has any of the following: energy, momentum, pressure, or stress, will produce gravity. Dauto (talk) 14:02, 30 October 2011 (UTC)

Yes; however, the gravitational field of its probability density function point cloud is so wide as to make it useless for predicting the outcome of gravitationally influenced events except in the most extreme boundary conditions. Dualus (talk) 23:13, 30 October 2011 (UTC)

Buying electronic components

If you buy some resistors, transistors or any other small electronic components, will some online shop send them through the normal post? Or do you have to pay relatively huge delivery costs for a component which only cost some cents (probably)? 88.9.210.218 (talk) 13:48, 30 October 2011 (UTC)

A lot of places charge relatively high postage and packing for small orders, while offering free delivery on much larger orders. I've recently bought some stuff from Bitsbox (in the UK) which sent them through the Royal Mail first class for only £1.50, but didn't give much information on the specs so it isn't really an option if you need the datasheet or anything particularly unusual. Places with more detailed information and wider choice tended to add more p&p. So, it depends on whether you're doing something basic that just needs a few cheap resistors and LEDs with vague properties, or whether you're doing more delicate work where you need to keep accurate track of voltage and current. (I got my batteries from Amazon, which was far cheaper than anywhere else) 86.163.1.168 (talk) 15:59, 30 October 2011 (UTC)

On ebay there are many component sellers, many with free postage via normal post. You will find that the minimum purchase is perhaps 10 items for around a dollar, or 1000 items for $10. Some places such as Hong Kong or Singapore have cheap postage. You will not be able to pay less than the cost of posting a small packet. Graeme Bartlett (talk) 10:59, 2 November 2011 (UTC)

Solar System

Is it possible that there are more than 8 planets on our solar system? or it has two suns? Do we know all the Big heavenly bodies thats rotating around the Sun? Is there any Papers written regarding this issues? — Preceding unsigned comment added by 203.112.82.128 (talk) 21:56, 30 October 2011 (UTC)

Wikipedia is a good place to start.--Aspro (talk) 22:01, 30 October 2011 (UTC)

Extra planets are possible but not likely. An extra sun would have already been found if it existed. Dauto (talk) 22:14, 30 October 2011 (UTC)

The possibility of extra planets in the solar system depends on your definition of "planet". 67.169.177.176 (talk) 22:55, 30 October 2011 (UTC)

If there were any other large bodies near the other planets, then we would able to detect the effects of its gravity on those planets. It is possible there are some large bodies out in the Kuiper belt or Oort cloud, though. See Nemesis (hypothetical star) and Tyche (hypothetical planet) for a couple of theories (although they aren't considered very likely to be true). --Tango (talk) 22:23, 30 October 2011 (UTC)

I asked because cracked.com says its one of the things science has no asnwers yet. — Preceding unsigned comment added by 203.112.82.128 (talk) 22:26, 30 October 2011 (UTC)

You appear very familiar with Wikipedia. Why are you asking these things here, instead of using this encyclopedia yourself? Do you want to be spoon fed?--Aspro (talk) 22:30, 30 October 2011 (UTC)

I agree with the OP that this is really not a polite or useful response. Please keep in mind the "be polite" guideline for people answering questions. The entire point of the Ref Desk is to help people with questions, not just say "look on Wikipedia" without providing any guidance as to where to look. --Mr.98 (talk) 23:44, 30 October 2011 (UTC)

Thanks!

Maybe because its possible that im not able to get the answers on my own, so im asking some wikipedians who i believe is more knowledgeable than me to provide some answers or atleast refer me to articles that can give me answers, im sorry if i wasted your time reading and answering my questions. — Preceding unsigned comment added by 203.112.82.128 (talk) 22:54, 30 October 2011 (UTC)

The definition of what constitutes a planet has recently been called in to question, so the number of planets has become controversial and indeterminate in the exact. Dualus (talk) 23:15, 30 October 2011 (UTC)

Take a look at the links that Tango gave; they are probably the sort of thing they were referring to. When you talk about things orbiting WAAAY out from the Sun (e.g. tens of thousands of astronomical units, where an astronomical unit is the distance from the Earth to the Sun), there is a lot of uncertainty as to what is out there. There isn't a lot of evidence to suspect there being a large planet or small sun out there, but apparently it's not totally impossible that one could be hiding in all the noise. --Mr.98 (talk) 23:43, 30 October 2011 (UTC)

We know that the mass of the Oort cloud is probably much larger than people thought it was thirty years ago. Dualus (talk) 04:05, 31 October 2011 (UTC)

See Nemesis (hypothetical star) for your second sun (which as indicated is just speculation). See Eris (dwarf planet) for what by right might be another planet; more generally planets beyond Neptune. Wnt (talk) 03:33, 31 October 2011 (UTC)

we are working on solar system akbar mohammadzade, as you know there are enough references which shows properties of solar system members , especially those formation model . some of shortages of this model will be corrected by our researches and exist observational data will be entered to model , together with new points and locations such as having some forecasting based on model.

Reconstruction of model will give it new abilities and help man in finding exoplanets , for this objective we need theoretical calculations and scientific data base ,using astrophysics concepts. we will show some characteristics of solar wind and mass increasing of Jovian planets . After publication of those reports I will be able to say them here. please guide and help us if you are specialist in COSMOGONY.--Akbarmohammadzade (talk) 08:54, 1 November 2011 (UTC)

Non-stiff motorcycle/bike helmets

Do helmets have to be stiff? It's clear that they have to protect against penetration of pointed objects and against abrasion, but couldn't they be just made of thick Kevlar covered by small pieces of hard plastic (but without forming a stiff ball)? — Preceding unsigned comment added by 88.9.210.218 (talk) 23:42, 30 October 2011 (UTC)

I think the MAIN thing they protect against is impact, not abrasion or penetration. For impact, stiff is definitely better. Vespine (talk) 00:15, 31 October 2011 (UTC)

I am not sure you are right about stiff being better. Stiff is often better because most soft materials won't distribute the impact force, But, wouldn't Kevlar protect against impact too? It seems to me this material is fantastic at distributing the force of the impact. 88.9.210.218 (talk) 00:20, 31 October 2011 (UTC)

Kevlar IS stiff AND already used in Kevlar#Armor including kevlar helmets; There is kevlar weave which is maybe what you are thinking of, such as used in Sailcloth#Kevlar and also now popular now in motorcycle clothing, but that is almost purely for abrasion protection, not impact resistance, so won't make a good helmet. Vespine (talk) 01:44, 31 October 2011 (UTC)

This is true.. while you could probably make a pretty good helmet out of kevlar weave- and it could work for multiple impacts- it would have to be pretty damn heavy. Stiff materials tend to be pretty kick ass for cheap, compact, one-use applications like motorcycle helmets and hardhats. Nevard (talk) 06:59, 31 October 2011 (UTC)

This brings to mind the classic 'hairnet' bicycle helmets from 30+ years ago. The article claims "This offered acceptable protection from scrapes and cuts, but only minimal impact protection". Remember that part of the point of a helmet is not just to distribute a force or withstand an impact, but to collapse on impact thus reducing the acceleration forces on the brain; something like polystyrene can do this well, while being very lightweight for its volume, necessary properties in a helmet. --jjron (talk) 14:50, 31 October 2011 (UTC)

Kevlar is neither stiff nor non-stiff: stiffness depends more on the form than on the material. Glass, steal, plastic, bamboo: all can be stiff or very flexible. 88.9.210.218 (talk) 22:27, 31 October 2011 (UTC)