Wikipedia:Reference desk/Archives/Science/2014 February 2

= February 2 =

Large body flying through the solar system.
Suppose a rogue planet with mass like that of Jupiter went thru the inner solar system at high enough speed to keep going and exit the solar system. Would it be true to say that, the higher the rogue planet's speed, the less disruption to orbits of planets like earth, mars and venus would be caused? Regardless, would orbits be severely deranged? thanks.76.218.104.210 (talk) 05:58, 2 February 2014 (UTC)


 * If it went perpendicular to the ecliptic and all the planets happened to be on the other side of the Sun, I suspect all it might do would be to permanently disrupt Earth's climate by altering its orbit, and cause mass extinctions by disrupting the asteroid belt, but s.o. would have to do the calculations. — kwami (talk) 06:57, 2 February 2014 (UTC)


 * Yes, in general, higher speed should mean less disruption. (Providing it doesn't actually hit anything that is!)  Gravity is the only significant effect it would have.  Gravity imparts an acceleration onto nearby object for as long as it's present - and the longer that acceleration goes on, the more velocities throughout the system are changed - and hence the more kinetic energy is transferred between the various bodies involved.  So as a general observation, the amount of energy that it would "rearrange" throughout the system would be in proportion to the amount of time that it was nearby - so the higher the speed, the less time it's transferring energy and the less "disruption" there is.  Of course a heck of a lot depends on when, where and in what direction this body was moving...and "disruption" is something of a fuzzy term...if all it did was drastically move the orbits of a few comets - then maybe we'd say that was considerably less "disruption" than (say) an 0.01% change in the Earth's orbital velocity - even if the energies involved were higher for the comets. SteveBaker (talk) 15:53, 2 February 2014 (UTC)
 * If you are really interested in stuff like this, you can try a gravity simulator like this one and see for yourself what happens. Not sure how easy it would be to set up a scenario like the one you describe, I personally haven’t played with one but I’ve heard it talked about in the context of a game I’ve started playing and it sounds like it should be possible. If not with the software I’ve linked, then with something similar. Vespine (talk) 02:50, 3 February 2014 (UTC)
 * The orbits may be temporarily disturbed, as the mass of the rogue planet nears the ecliptic (we'll assume the planet passes very close to the sun for simplification) the planets will "feel" more gravity and their orbits briefly contract, ever so slightly, but once the planet is far off, the planets would resume "feeling" the same gravitational force as before and settle back into their original orbits. I imagine the greatest effect would be to change the ellipticity of the planets' orbits. — TimL &bull; talk 09:10, 3 February 2014 (UTC)
 * No, that's off, TimL. The perturbed orbits would be under the influence of the original planets as well all the time, the effects of the acceleration they underwent due to the intruder would not simply disappear.  There wouldn't be any further disruption by the intruder once it passed, but there are no preferred orbits to snap back to, and once in motion stays in motion. μηδείς (talk) 19:24, 3 February 2014 (UTC)
 * Playing a game called Kerbal Space Program has taught me a few not very intuitive things about orbits. Orbital changes are most effective by aiming force (thrust) in the prograde or retrograde vector at the apoapsis of the orbit. Many orbital maneuvers, such as the Hohmann transfer orbit are performed in this way. In particular at the apoapsis or a very eccentric orbit, the orbital velocity can get very low, so that minimal thrust in any direction can have an extreme effect on the orbit. The orbits of the planets in our solar system are not very eccentric, but my point is that a momentary force, not necessarily even a very large one, can have a big effect on an orbit from which the body will certainly not simply "recover" after that force is gone. Vespine (talk) 02:53, 6 February 2014 (UTC)

most ancient arrow poison
Hi,

I am trying to find reliable information on the most ancient arrow poison ever used (maybe from chemical analasys of ancient arrows found?). The article chemical warfare mentiones use of arrow poison by the San people 10000 years ago, but with no reference. I would appreciate any information. Thanks! 2.55.131.28 (talk) 07:15, 2 February 2014 (UTC)


 * The article on the San people mentions Diamphotoxin as the poison used. --Cookatoo.ergo.ZooM (talk) 10:13, 2 February 2014 (UTC)


 * yes, but since when? 2.55.127.242 (talk) 12:39, 2 February 2014 (UTC)


 * If you include biological warfare, dipping arrows in feces might go back much further. The people doing so wouldn't have needed to know about bacteria, or even that it would tend to cause the enemy to become infected.  They might have done so just as a way to humiliate the enemy. StuRat (talk) 18:36, 2 February 2014 (UTC)


 * I am curious whether there might be a telltale sign on arrowheads like grooving that indicates they've been modified for poison. μηδείς (talk) 20:48, 3 February 2014 (UTC)


 * Unfortunately, grooves can also be there to help attach the arrowhead to the arrow. StuRat (talk) 23:25, 3 February 2014 (UTC)
 * Yes, but attachment grooves have only one location and orientation. Additional grooves would be suspect. μηδείς (talk) 17:41, 4 February 2014 (UTC)

How did Newton derived his law of gravitation?
How did Newton derived his law of gravitation? 182.66.52.106 (talk) 14:02, 2 February 2014 (UTC)
 * The history of how Newton's law of gravitation came to be is somewhat under dispute. See Newton's law of universal gravitation.  Red Act (talk) 14:29, 2 February 2014 (UTC)


 * Newton wrote down exactly how he derived it! It's called Philosophiae Naturalis Principia Mathematica, and it is one of the most important physics books ever written.  Fringe historiographers can debate all they like about attribution, or discuss prior work that influenced Newton, but there is no reason to doubt that Newton wrote this book; so to answer the question "how did Newton derive his law of gravitation," there is no better source than Principia.
 * If you choose to read the Principia (for example, in English translation), you can see exactly how Isaac Newton presents the derivation of universal gravitation. He begins with philosophical assertions.  I paraphrase them thusly: natural laws ought to be consistent throughout the universe, and natural law directly corresponds to observable phenomena, and there are no hidden variables (Newton calls these "intensions").  Next, Isaac Newton presents a data table of observations concerning the Galilean moons - the four largest moons of Jupiter that are easily visible from Earth - even using 16th-century optics.  Newton writes some mathematics to show that the conclusions of Keplerian motion are consistent with those observations.  Finally, Newton writes a mathematical expression that relates the Keplerian motion to a central force law of second order - which he already explored mathematically in Book 1.  Over the next few hundred pages, he explores these consequences in greater depth.  As Isaac Newton writes in his introduction to Book 3 of the Principia, a commonly-educated man might not have time to read and study all of Principia - there are so many propositions - but anyone sufficiently familiar with the most important principles and definitions - what we today call Newton's laws of motion - can skip directly to Book 3 and study his derivation of the mechanics of gravity.  Nimur (talk) 17:11, 2 February 2014 (UTC)


 * My understanding is that the history is a little more complicated than that. As I understand it, Newton invented calculus and used it to work out the law of gravity and its consequences.  But when he wanted to publish it, he felt that people wouldn't believe results obtained using such a newfangled method, so for the book he worked out proofs for everything using classical geometry, and completely avoided any mention of calculus.  The result is that the book is much more complicated than it really needs to be. Looie496 (talk) 17:32, 3 February 2014 (UTC)