Wikipedia:Reference desk/Archives/Science/2013 September 30

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September 30

Where is East?

My bedroom faces East. In spring the sun rises on the left side of the window. In autumn on the right. It is quite a significant shift. My guesstimate on degrees of a circle would be between 60-90 degrees. I know North changes slightly. My questions are these. What is the range that sunrise covers? Does latitude make a difference? Where is East on the map? And how do aircraft navigate - as East on the map, is actually not a fixed direction? 31.25.4.14 (talk) 09:35, 30 September 2013 (UTC)

A modern nautical compass rose

True east is 90° CW from true north (geodetic north) and does not change. Magnetic east, 90° CW from magnetic north, does change as the North Magnetic Pole slowly moves. The difference (in degrees) between the two is called magnetic variation. Nautical charts include a compass rose which shows both true and magnetic direction, and gives the numerical value of that location's variation in a particular year and the rate of change of that variation to allow for use in subsequent years. "The sun rises in the east." is a general statement, and is not strictly true. (It certainly does't rise in the west, but it doesn't rise due east everywhere on every day of the year.) See Sunrise#Location on the horizon. This sun calculator may be of interest to you. -- ToE 10:20, 30 September 2013 (UTC)

Only around the March and September equinox will the sun rise approximately in the east, between March and September, it will be more to the north, between September and March more to the south, as the sun calculator given by ToE demonstrates. Ssscienccce (talk) 10:45, 30 September 2013 (UTC)

And yes, latitude does make a difference. For any given day, the sun will rise farther from east for locations with greater latitude, but even on the equator it will only rise due east on the equinox. We are currently just one week after the autumnal equinox, and the angles shown in the sun calculator are not large, so you may wish to advance the calculator's date. If you bring it well into the northern winter and choose an extreme northern latitude with only minutes of sunlight per day, you will see that the sun is rising and setting minutes apart in the south. -- ToE 10:56, 30 September 2013 (UTC)

And here is another interesting calculator. 31.25.4.14 geolocates to Harrogate with a latitude of 54°, so the calculator shows that your solstice sunrises are nearly northeast in your summer and southeast in your winter. -- ToE 12:00, 30 September 2013 (UTC)

Truly fascinating. Thank you. 31.25.4.14 (talk) 08:52, 1 October 2013 (UTC)

Why do planet have eliptical orbit?

If sun's gravity at a planet has same in every direction then why a planet moves in eliptical orbit? — Preceding unsigned comment added by 101.217.215.28 (talk) 12:08, 30 September 2013 (UTC)

Elliptic orbit

It would be circular if the planet started at exactly the right speed perpendicular to the Sun, but it doesn't for any of the planets. Kepler orbit has some complicated maths to show how an elliptic orbit happens. Here is a very loose argument not showing why it exactly becomes an ellipsis: Imagine the planet in the drawing is moving down-left falling closer to the Sun for a while. It picks up speed and when it's to the left of the Sun, the speed is maximal and so high that it starts moving further away from the Sun and slow down. At the right edge of the ellipsis, the speed is minimal and so slow that it starts falling closer to the Sun again and increase speed. PrimeHunter (talk) 13:49, 30 September 2013 (UTC)

Note that the planet only moves perpendicular to the Sun at the left and right edge of the ellipsis. At the left edge the speed it too high for a circular orbit so it moves further way. At the right edge the speed is too low for a circular orbit so it moves closer. PrimeHunter (talk) 14:00, 30 September 2013 (UTC)

The orbit is stable if the sum of the gravitational and kinetic energy remains the same throughout the orbit. With a perfect balance like that, no energy is required to be added or removed from the system, so the planet keeps doing the same thing over and over. There is no reason why the planet can't move slowly when further from the sun, and faster as it gets in closer - so long as the sum of gravitational and kinetic energies doesn't change. Such orbits can be perfectly stable over billions of years. A circular orbit certainly has that property - but so does an elliptical one. Statistically, it's extremely unlikely that a planet would happen to be in a perfectly circular orbit - so an ellipse is more likely. Also, the presence of more than one planet in a solar system perturbs orbits slightly - so a circular orbit wouldn't remain perfectly circular for very long. SteveBaker (talk) 14:01, 30 September 2013 (UTC)

In the absence of any perturbing third objects, is it even possible for a very small passing object to be captured into a perfectly circular orbit by a very large object? If yes, under what condition on velocity etc.; if no, why not? Thanks. Duoduoduo (talk) 15:46, 30 September 2013 (UTC)

In the absence of other objects, and in the classical case, two objects either have a positive total energy, or a negative total energy (where total energy is the sum of potential and kinetic energy). Since energy is conserved, this will not change (unless the two smash into each other ;-). In the fist case, they are bound, in the second they are not. In the relativistic case, the system can lose energy via gravitational waves, so in theory they can capture each other. But that is a very small effect. In practice, capture will (nearly) always require a three-body interaction. --Stephan Schulz (talk) 19:48, 30 September 2013 (UTC)

It's worth pointing out to the OP that the diagram is nowhere near to scale for an actual planet. For example, the earth only varies a few percentage points between aphelion and perihelion. The diagram is closer to describing the orbit of a periodic comet such as Halley's - although for a comet it's probably not extreme enough. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:10, 30 September 2013 (UTC)

As Baseball Bugs points out, the orbits of the planets are actually quite close to being circular, so the question should be why that is the case. So, why aren't the orbits of planets a lot more elliptical? The answer is that the solar system is actually unstable, an initial state where you have large eccentricities would lead to collisions and planets being ejected from the solar system. This is precisely what happened during the formation of the solar system. What you are then left with at the end of the planet building phase is a more stable configuration where the orbits are almost circular and the ratios of the orbiting periods are such that resonaces are minimized. So, the perturbations on the very long term will actually steer the orbits toward becoming more circular, but they won't make them exactly circular. The solar system is still unstable, but on a time scale of billions of years.

The mechanism here is similar to how thermal equilibrium is approached in many body systems. In a state of thermal equilibrium a particle will still experience thermal fluctuations, but you will have equipartition of energy, the energy transfer from one particle to the rest of the system another will then average out to zero on the long term. Count Iblis (talk) 16:32, 30 September 2013 (UTC)

I think the person asking wants a more basic answer, so let's take a try at being simple: At its closest point to the sun, a planet is a lot like a ball you throw up into the air. Even though it is at its lowest point, it is moving quickly on a line that will take it further away from the sun. Its course bends from that line because of the sun's gravity, and eventually, like a ball tossed in the air, it runs out of energy and starts falling back again. That point where it starts falling back is the furthest point from the sun, but because the planet is still moving perpendicular to the sun at that point, it's going to miss it on the way in. After the planet finishes falling back, it has the extra speed it started out with all over again (this is conservation of energy). Now it happens, mathematically, that this curve is almost an ellipse - (Kepler's law says exactly, but the theory of relativity corrects this a little). In a sense, that's just how the math works out, but there's a deeper meaning too: an ellipse (and more precisely the path corrected for relativity) are shapes that allow the combined angular momentum of planet and sun spinning around one another to remain the same as the planet moves in and moves out. Like an ice skater going into a spin, the planet has to change the angle it has relative to the sun fastest when it is closest to the sun, slowest when it is furthest away. The near-ellipse allows for this change to match the change in the planet's speed as it goes in and out. The only way that the ellipse turns out to be a simple circle is if at its moment of being closest to the sun, the planet has exactly the right speed to keep swinging around in a circle at the same speed. If you could "hit the brakes" on the planet for a moment, it would start falling with that being the highest point in a new ellipse, and if you could "hit the gas" we'd be back to what I described above. Wnt (talk) 19:50, 30 September 2013 (UTC)

The way I read this question the OP is asking why the orbits don't have the same symmetry as the potential that generates the force. - That is the Potential is spherically symmetric but the orbit isn't. The answer is that the initial conditions aren't spherically symmetric and they also have a say on the path of the orbit. But notice that the set of all possible orbits does posses spherical symmetry even if no single orbit does. Dauto (talk) 16:06, 1 October 2013 (UTC)

Work done by a person carrying a bag on his head

What is the work done by a person while moving and carrying a bag on his head? Scientist456 (talk) 12:29, 30 September 2013 (UTC)

Should be none at all theoretically disregarding friction etc. It's no different from sticking a load onto a trolley and giving it a push to send it away. Actually many people find this a very convenient way to carry loads. Dmcq (talk) 12:37, 30 September 2013 (UTC)

I strongly disagree. It's certainly not true that this is no different than using a trolley (although you're right about the fact that a lot of people prefer to carry loads like this).

As you walk, your head bobs up and down a little (see animation at right). That requires you to lift the load against gravity (expending muscular chemical energy to do so) and lower it again (requiring you to cancel the resulting kinetic energy with more muscle power)...repeated with every step you take. Pushing it on a trolley leaves it at the same height - so the only additional expenditure of energy is the friction of the wheels. I'm not sure whether one or the other is more work. There are much more complicated things at play if you're moving around on uneven ground - so using your head instead of a trolley might well make sense. But there is certainly a profound difference.

In reality, it's even more complicated because a human body isn't like a robot. In theory, a robot with a hydraulically powered arm could hold it outstretched with a heavy weight on it for years without expending any energy whatever to do so. But for a human, that kind of activity is exhausting! So merely placing the weight on someone's head and having them stand still must incur some energy consumption beyond the extra energy required for head-bobbing during walking.

The answer here is very complicated. We shouldn't over-simplify it. What's needed for a good answer here is some kind of study into the biometrics of the thing.

SteveBaker (talk) 13:42, 30 September 2013 (UTC)

(ec) No. How would you walk without lifting your feet? The friction would be enormous if you tried to slide them. The work is the same as if you carried it any other way. The real advantage is that the center of gravity of the load is roughly the same as the center of gravity for the person. This counters the effect of having to lean into the load so you don't topple over. 196.214.78.114 (talk) 13:44, 30 September 2013 (UTC)

Check out this clip to see what I mean. http://www.youtube.com/watch?v=lV-iP1jSMlI 196.214.78.114 (talk) 13:49, 30 September 2013 (UTC)

Old studies claimed that it was more efficient than other ways of carrying loads, a recent publication suggests that it may be less efficient, and is used mostly to traverse difficult terrain (although that seems strange to me, considering the odds of dropping it). I would have thought that it had something to do with keeping the center of mass "centered" so it's supported by the skeleton instead of the muscles, but primarily with the fact that you don't need "grip" to hold the object. Without straps, ropes or a suitable shape with handles, it's hard to support large objects with your hands or arms. But the old theory that they could carry 20% of their body weight without spending more energy seems in doubt. Here are some studies mentioned, one that found Nepalese porters carried loads more efficiently than African women. Ssscienccce (talk) 14:30, 30 September 2013 (UTC)

There is an article Carrying on the head about this. It doesn't describe there but when doing this one normally moves the head in a level way like those Russian dancers so the weight doesn't go up and down much. If we had sinews that were as good as kangaroos I guess even that would be unnecessary as we'd get most of the energy back after each step. Dmcq (talk) 15:07, 30 September 2013 (UTC)

Why is the speed of light in vacuum not relative?

Suppose if the earth were moving toward a star, the light from that star should seem faster than if the earth were moving away from that star. But it does not happen, why? If something is moving with a speed of 100,000 km/s, then it still experiences the speed of light to be 300,000 km/s. Why is it so? Why don't it experience light to be moving at 200,000 km/s (=300,000 - 100,000) or 400,000 km/s(=300,000 + 100,000)? Britannica User (talk) 13:32, 30 September 2013 (UTC)

The speed of light is constant to the observer. What can vary is the frequency, i.e. the light-wave version of the Doppler effect. If a galaxy is approaching, its light should be blue-shifted. If it's receding (the typical situation) it should be red-shifted. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:44, 30 September 2013 (UTC)

Eh? What does that have to do with it? SteveBaker (talk) 13:51, 30 September 2013 (UTC)

I'm giving you the explanation my Physics teacher gave. Feel free to make corrections. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:05, 30 September 2013 (UTC)

Read Special relativity, which explains some of the consequences of the constancy of the speed of light. But, in simplest terms, every observer does measure the speed of light at the same speed, regardless of how fast they are moving. This seems counterintuitive to people who have only experienced measuring the speed of objects like cars and trains and things, but none-the-less it is how the universe actually works. The consequences of this lead to several apparent paradoxes, such as concepts like the relativity of simultaneity and the like, but again these are not paradoxes except in the sense of "I don't experience this so it doesn't make sense" as opposed to "this isn't how experimental observation shows the Universe to work." Concepts like special relativity are a good demonstration of why relying on ones own senses and "common sense" and not on the collective results of experimental science often leads one astray. --Jayron32 13:50, 30 September 2013 (UTC)

I'm not sure we know why this is. Physics is relatively poor at answering "Why?" questions. We know that the universe operates in this manner...but why it does that is hard to say. You can construct arguments like the fact that photons have a zero rest-mass and a measurable mass when in motion, and because of the laws of relativity, that means that they have to be moving at the speed of light in any reference frame...but that only turns things around from "Why is the speed of light constant?" to "Why do photons have a rest mass of zero?" and "Why do the laws of relativity hold as true?". There doesn't appear to be a root cause - it is simply what we've observed in countless careful experiments and measurements of how the universe appears to operate. We know that the universe would be a drastically different place if this were not true. We can probably make a valid claim that life would not exist if it were not true. Hence we can probably invoke the Anthropic principle to say that if this were not true, we wouldn't be here to observe it - so it has to be true...but that's always considered a weak explanation for such things. SteveBaker (talk) 13:51, 30 September 2013 (UTC)

The "why" of any fact about science can sometimes be determined by further experimentation. Why the speed of light is constant is the OP's question. My question would be why is the speed of light what it is, i.e. about 186,000 miles per second? What is it about the nature of light that determines that that is its speed rather than some significantly larger or smaller number? ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:04, 30 September 2013 (UTC)

The speed of light is determined by two physical constants: Vacuum permittivity and Vacuum permeability. But then the question is why are these constants what they are. (And it can be asked which is fundamental - c or the other two?) I think Maxwell proved the relationship between them and the speed of light. Bubba73 ^{You talkin' to me?} 02:14, 1 October 2013 (UTC)

What is this thing about science not answering "why" questions? It's never been true or made any sense. Why is the sky blue? Why don't oil and water mix? Why do most of the stars move as though affixed to a sphere, except for a few (the "wanderers", Greek planetes) that move in complicated patterns in a narrow strip called the ecliptic? Science answers most of the "why" questions that early people probably had about the world.

There's no reason velocities shouldn't behave as they do. Draw a horizontal line. Draw a second line crossing it with a slope of 1/2. Draw a third line with a slope of 1/2 using the second as a base. What is its slope relative to the first? The answer is not 1, but 4/3. You can work out that in general the combined slope is (m + n) / (1 − m·n). That looks a lot like the "velocity addition formula", and that's not a coincidence. If you had used angles instead, you could have just added them. The angle version of speed is called rapidity. You're just measuring speed in a funny way and then complaining that you get weird results. Yes, in a way, it's a mystery why anything is the way it is, but not in a way that's relevant to this question. We actually understand the nature of speed pretty well. -- BenRG (talk) 20:53, 30 September 2013 (UTC)

I would mostly agree with Jayron. You can go a bit further and argue that any local theory of physics where things that will happen here in, say, one second from now isn't going to depend on what is happening now elsewhere at arbitrary large distances, must have this property. Count Iblis (talk) 14:40, 30 September 2013 (UTC)

The particals of light move back and forth many time at time and stabilize on the future object , includ the return movement in time , Thanks Water Nosfim — Preceding unsigned comment added by 81.218.91.170 (talk) 07:23, 1 October 2013 (UTC)

Also note that sub-light speeds don't add and subtract as you may expect. Relative velocity#Special relativity theory shows the formula $v_{\mathrm {BA} }={\frac {v_{\mathrm {B} }-v_{\mathrm {A} }}{1-{\frac {v_{\mathrm {A} }v_{\mathrm {B} }}{c^{2}}}}}$. If two cars drive towards eachother at each 100 km/h from the reference frame of the road then each car sees the other approaching at ${\frac {100{\text{km/h}}-(-100){\text{km/h}}}{1-{\frac {100{\text{km/h}}\times (-100){\text{km/h}}}{c^{2}}}}}$. c = 1079252848.8 km/h is so large that the result becomes 199.9999999999983 km/h. Humans don't notice the difference from 200 km/h but advanced instruments can measure relativistic effects at Earth speeds. If we consider the lights from the two cars instead of the cars themselves and plug the speed c into the same formula then we get (c - -c)/(1 - (c × -c)/c²) = (c+c)/(1+1) = c. PrimeHunter (talk) 14:45, 30 September 2013 (UTC)

It's very surprising to people but in fact we don't live in a euclidean world. So it makes sense that any intuition based on a expectation that the world is euclidean might turn out to be incorrect. That includes the intuitive - but incorrect - simple addition of velocities the OP thought should apply - but doesn't. Ultimately, the geometric nature of the universe must be decided experimentally - not intuitively - and the experiments have been done. Turns out the universe is not euclidean. Dauto (talk) 21:00, 30 September 2013 (UTC)

I don't think the word Euclidean is on point here. Special relativity operates in flat spacetime, which is Euclidean, or at least "pseudo-Euclidean" "semi-Euclidean", with the "pseudo" "semi" part referring to the fact that the time component of the metric has the opposite sign from the spatial part, ds²=dx²+dy²+dz²−dt². Euclidean geometry doesn't talk about time anyway. So sure, our intuitions are off, but not because they're "Euclidean". --Trovatore (talk) 18:48, 2 October 2013 (UTC)

It's filtered back to me that it might be "semi", not "pseudo", as in "semi-Riemannian geometry" (in fully Riemannian geometry, the metric tensor is positive definite, whereas in semi-Riemannian geometry, it may not be. --Trovatore (talk) 20:52, 2 October 2013 (UTC)

Why do razor blades dull so quickly ?

They are steel, after all, and are only used to cut through hair, not corundum. I can think of a couple possible explanations:

1) The manufacturers intentionally use a soft steel, so they will dull quickly, and you will need to buy replacements.

2) The blades oxidize, and this dulls them. The blades often seem dull even with no sign of rust, so that seems to contradict this explanation, unless rust can dull blades before it becomes visible.

StuRat (talk) 17:44, 30 September 2013 (UTC)

My experience is that expensive razor blades can last for quite a while (dozens of uses), but cheap ones are only good for one or two uses (if that). Looie496 (talk) 18:14, 30 September 2013 (UTC)

I remember hearing that it is corrosion, with the advice to dry the blade with a towel after use. I've done that since and I feel like it helps, but I never kept track of uses or anything so it is subjective. I'll second the advice on cheap blades. I once decided to try a pack of cheap disposable razors. They could barely make it through one use, and they started out at the point where I would start considering tossing a nice blade. If anyone can find good information on making a blade last longer, I would greatly appreciate it - I don't get too many uses (certainly not dozens) out of a nice blade either. Katie R (talk) 19:22, 30 September 2013 (UTC)

You could try this approach, which (if it works) is probably a variant of stropping or honing. Assuming that shaving can bend or fold the edge of the blade, this would straighten it again. Ssscienccce (talk) 19:46, 30 September 2013 (UTC)

You could also try a pyramid surrounded by crystals under the full moon. (Disclaimer: It won't actually work but you'll have fun trying.) -- Jack of Oz ^{[pleasantries]} 20:13, 30 September 2013 (UTC)

Seems legit - if you can get a few dozen uses out of a blade, then only using it under the full moon means it will last for years. Katie R (talk) 11:36, 1 October 2013 (UTC)

Or use baby oil. One guy claims his Mach3 lasted for three years... Ssscienccce (talk) 21:31, 30 September 2013 (UTC)

If oil works, that supports the idea that oxidation is the problem, as oil is known to prevent that by keeping the metal away from air and water. StuRat (talk) 22:18, 30 September 2013 (UTC)

A good razor is honed to a very fine edge, I believe the sharp edge is typically fractions of a micron thick, it doesn't matter how hard the steel is at that thickness, the edge will warp and wear. Even very high quality straight razors need to be frequently honed and sharpened. Vespine (talk) 22:46, 30 September 2013 (UTC)

My experience agrees with Looie's that good quality blades last very well. Compared to my vaguely-remembered experience from say 20 years ago. I will try the baby oil idea but it sounds unlikely. Wanderer57 (talk) 23:26, 30 September 2013 (UTC)

Any idea what is different about the quality blades ? StuRat (talk) 05:08, 1 October 2013 (UTC)

I think the biggest factor is going to be the quality of the edge. Like I mentioned above, a cheap disposable starts out feeling like a better blade that I've already put noticable wear on. If the edge doesn't start out as sharp then it won't take as long to get to an unusable point. Katie R (talk) 11:40, 1 October 2013 (UTC)

For double edge blades, a non-stick coating is applied and baked at 660°F (350°C) for twenty minutes (according to "how it's made"). Maybe that is only used on more expensive blades? There was also a vacuum treatment to "pull the chromium to the surface"?? And a four stage heat/cold treatment. Ssscienccce (talk) 13:14, 1 October 2013 (UTC)

What definitely will dull your razor is dry shaving. At my first job I sometimes overslept on monday morning leaving me no time to shave, so I kept a razor in my briefcase for emergencies. But without water or other lubricant I could feel it getting duller with every stroke. It's not just about friction and abrasiveness, dry keratin is also 2.5 times harder than wet keratin. Read more here

Regarding corrosion: Standard stainless steel (304) will not take an edge; for a sharp edge you want more carbon and less chromium (and hardening by heat treatment). But less chromium means less rust resistance and less toughness, more carbon means sharper but more brittle, so in conclusion you want a steel that takes and edge that is sharp enough, but not sharper than needed because that comes at the expense of faster dulling and corroding. Ssscienccce (talk) 13:14, 1 October 2013 (UTC)

If soap cleans by removing sebum oil from your skin, and the palms of your hands don't have sebaceous glands, which are the glands that produce sebum oil, does that mean soap never cleans the palms of your hands?

If soap cleans by removing sebum oil from your skin, and the palms of your hands don't have sebaceous glands, which are the glands that produce sebum oil, does that mean soap never cleans the palms of your hands? If soap still cleans the palms of your hands, how does that work despite the absence of sebum oil of the palms of your hands? Rebel Yeh (talk) 19:41, 30 September 2013 (UTC)

Soap is a like detergent (strictly, it's a "Surfactant"). It reduces the surface tension of the water and allows it to flow more easily into small spaces and to mix more easily with oils - and also to encapsulate small particulates. That helps to remove whatever it is that's adhering to your hands...not just sebum, but any kind of oil, fat or other similar residue. SteveBaker (talk) 20:09, 30 September 2013 (UTC)

Just to clarify a bit, soap doesn't clean by removing sebum. Soap cleans by allowing the water to dissolve more substances than it otherwise would. (this is a gross oversimplification, but right enough for a discussion of this level). Sebum may be one of those substances, but there's lots of other things of greater concern than sebum. Indeed, preferably, you want to keep your sebum, as removing that leads to problems with dry skin. Good hand soap strikes a balance between removing dirt and leaving behind your own oils. --Jayron32 20:37, 30 September 2013 (UTC)