Wikipedia:Reference desk/Archives/Science/2010 November 13

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November 13

Cells

How does the repaired or changed stem cells replace the old or damaged cells once injected? Do they wait for the damaged cells to die off? —Preceding unsigned comment added by 71.137.241.60 (talk) 02:09, 13 November 2010 (UTC)

See Stem cell treatments and also follow links from that article. --Jayron32 02:28, 13 November 2010 (UTC)

Carrying a tray is not work...

When I carry a tray across a room, my arms don't do any work on the tray. I totally understand this. However, I think that my legs are doing work on the tray. My walking legs provide the horizontal force that move the tray. Is this correct or not, and if not, why not? Lova Falk talk 11:10, 13 November 2010 (UTC)

I think that's correct yes. Your arms are exerting a force on the tray though which counteracts the force of gravity. ScienceApe (talk) 12:55, 13 November 2010 (UTC)

Work = force x distance. If there is not anything moved, then Work = force x 0 = 0. --Chemicalinterest (talk) 13:15, 13 November 2010 (UTC)

But the tray is moved horizontally across the room. So the distance is a couple of meters. Lova Falk talk 13:26, 13 November 2010 (UTC)

Yeah but like you said, your legs are doing the work, not your arms. ScienceApe (talk) 13:30, 13 November 2010 (UTC)

Why do my arms get tired of carrying a tray before my legs then?--Shantavira|^{feed me} 13:34, 13 November 2010 (UTC)

1. Because the term work, like theory, means something different in physics from that in general usage. Also because arms are wimps compared with legs, for most people. Imagine Reason (talk) 13:39, 13 November 2010 (UTC)

(edit conflict) Because the muscles in your arms have to exert a force to counteract gravity. In physiological terms, this is "working" and consumes chemical energy, though no "work" is done by your arms in the physics sense. You legs are doing work in the physics sense because your centre of gravity moves up and down as you walk, but leg muscles are accustomed to this and don't tire easily. Dbfirs 13:42, 13 November 2010 (UTC)

As I understand it, your arm muscles are constantly flexing to keep the tray in place. The tray moves up and down all the time, and its this that makes your arms feel tired. 92.29.122.31 (talk) 17:50, 13 November 2010 (UTC)

Folk are confusing "force" with "work".

A magnet exerts a force - it does not "do work". Your fridge magnet will stay stuck to your fridge until hell freezes over without "running down" because it doesn't need to expend energy to do it. Energy is produced or consumed when something that's exerting a force moves. So when you peel the magnet off of your fridge, it costs your muscles energy to do it. When you release the magnet from a half inch away from the fridge, it releases that energy by moving back onto the fridge - and the energy you added with your muscles is turned into heat and sound as it snaps back onto the metal. The magnet itself doesn't HAVE energy to give away - it's not like a battery or a wound-up clockwork toy.

The clockwork toy is a great analogy. If you wind up a clockwork toy car and place it on the table with your hand in front of it, you can feel it pushing against your hand. But until you move out of the way and let the car shoot across the table, it's its spring won't ever run down. It'll stay there with the spring wound up, actively pushing against your hand, until you release it - even if that takes an hour, a week or a thousand years. Once the car starts moving, it gains "kinetic energy" (the energy of motion) and gradually loses the energy in the spring.

The situation is exactly the same as with another common force...gravity. If you place a tray onto a table, the tray and the table are exerting forces on each other - the tray is being pulled downwards by gravity - and the table has interatomic forces that resist motion and press upwards onto the tray to prevent it from moving. Again - there are lots of forces being applied here - but no energy is being produced or consumed in the process because nothing is moving (technically: because nothing is accelerating). The tray will stay there, resting on the table forever without the table or the tray "running out of energy" because they are doing that. But if you exert chemical energy from your muscles to pick up the tray - you are giving it energy because you are working against the force of gravity...and when you drop it onto the floor, this energy will be released as it falls, turning into kinetic energy (motion) and then into heat and sound as it hits the floor. Cuddlyable3 (talk) 14:16, 13 November 2010 (UTC)

"Work = force x distance" is WRONG! The correct formula is "Work = force . distance" as it is the Dot product of two vector properties that yields a scalar result and not in this case the cross product.

It's also helpful to note that Work is energy and so has no direct relationship to power, which is money. Hcobb (talk) 15:26, 13 November 2010 (UTC)

Your arms do a small amount of positive work while accelerating the tray against inertia (applying a force), "dot" the distance where that force is applied. But you do an equal amount of negative work decelerating the tray while it continues to move in the same direction when you get to the other end of the room (the dot product is negative). (This negative work makes a lot more sense if you consider a hybrid car with regenerative braking!)

Now the reason why your legs don't get credit for holding up the tray is that a better designed tray carrier would just slide frictionlessly across the floor, holding it up the whole time without consuming energy.

But I don't give my legs credit for the vertical position of the tray, I want to give them credit for the horizontal movement of the tray. Lova Falk talk 20:19, 13 November 2010 (UTC)

There is work done by your legs to carry the tray from one position to another horizontally. If the tray weighed 100 kilograms (220 lb), your legs would notice that more (they would notice if you just stood there, too, but they would notice even more if you tried to walk). WikiDao ☯ (talk) 20:35, 13 November 2010 (UTC)

We do have an article on the physics of this: Work (physics), which says:

$W=F\cdot d$

where

${\vec {F}}=m{\vec {a}}$

and m is Mass.

What you feel when you are just holding the tray is its Weight, or the force of gravity. If you were carrying the tray in a gravity-less environment, so that its weight was zero but its mass was still say 100 kg, you would find it more difficult to move it "horizontally" than if its mass was only 10 kg. See also Mass versus weight. WikiDao ☯ (talk) 21:01, 13 November 2010 (UTC)

... and in physiological terms, your legs need to do work both to accelerate and decelerate the tray. There is no way that the kinetic energy can be recovered, so work has to be done to bring the tray to rest again, even though the total work done in physics terms is zero. The work done appears (at least partly) as heat in the muscles of the legs and to some extent in the back and shoulders. Your arm muscles would also do some work if you held the tray out in front of you, but obviously not if the tray was held by a harness round your neck. Dbfirs 21:22, 13 November 2010 (UTC)

I see no reason why muscles couldn't do the equivalent of regenerative braking, converting some of your kinetic energy back into ATP. I think they don't, but it's not ruled out by the laws of physics. Likewise, you could gain energy carrying a heavy object downhill. The long and short of it is that our bodies suck at this task. We lose a lot of energy (as heat) unnecessarily. Otherwise we wouldn't get tired. -- BenRG (talk) 00:49, 14 November 2010 (UTC)

To this day it mystifies me why muscles didn't evolve with a "hold" mode that would require no energy to use; though biological uses for it would be rare you'd think there'd be some. I wonder if the myosin heads all locked down in one static pattern for some hours, that structural flaws might damage the muscle? Wnt (talk) 18:58, 13 November 2010 (UTC)

Birds lock their claws onto a perch, and some animals sleep standing up by locking their legs, but I think this is joints not muscles that achieve this effect. Dbfirs 21:22, 13 November 2010 (UTC)

You expend plenty of energy moving the tray around, but that's because we're pretty inefficient at that sort of thing, and that's not the same as doing work (in the physics sense of the word). There's a lot of frictional forces involved, and all that energy ends up becoming heat. You can think of the amount of work done on the tray as how much energy you've added to it. If you carry the tray around and it ends up back where it started, you haven't actually changed the tray's situation so you haven't done any work on it (even if you expended energy). If you lift the tray up to a higher position, you have done work on it since now the tray has more potential energy than it did before. If you throw the tray, you've done work on it by giving it kinetic energy. Rckrone (talk) 04:08, 14 November 2010 (UTC)

Perhaps this is in part a biology type question? When one is carrying a tray, even though the tray is not moving up or down visibly, the actin and myosin fibres in your skeletal muscles are actually engaging and disengaging many times a second and expanding a large amount of energy to do so. As such, you arm is getting pulled down very minutely and pulling itself back up again very minutely to counteract the force of gravity and thus you ARE actually doing work by carrying a tray. Muscles such as smooth muscle work by a slightly different mechanism so this same engaging and disengaging does not happen. Thus if your arm was made of it you would THEN not be doing any work, however this type of muscle fibres reacts more slowly and your controllable muscles are not made of them. -- Sjschen (talk) 00:02, 16 November 2010 (UTC)

Efficiency of firearms

What is the efficiency of the chemical energy contained in modern propellant to the kinetic energy it imparts to a bullet? ScienceApe (talk) 12:54, 13 November 2010 (UTC)

A firearm is effectively a single-stroke Internal combustion engine or heat engine and efficiency is described by thermodynamic cycles. None of these cycles exceed the limit defined by the Carnot cycle which states that the overall efficiency is dictated by the difference between the lower and upper operating temperatures. Most engines have energy efficiency of about 18%-20% which I expect is similar for firearms. Cuddlyable3 (talk) 13:56, 13 November 2010 (UTC)

Our article on physics of firearms claims roughly 30% efficiency for what is nominally a very efficient cartridge. It's actually not too difficult to calculate for yourself if you know the amount of gunpowder used along with the mass and muzzle velocity of the bullet. The lone external link from that article here puts the energy content of powder (including binders and additives) at about 1.3 million foot-pounds per pound of powder (about 4 kilojoules per gram); that's equal to about 190 foot-pounds or 260 joules per grain of powder. Multiply that energy density by the mass of powder in the load and that tells you how much energy goes in.

Next calculate the muzzle energy of the bullet. (That's one half of the mass of the bullet, multiplied by the square of the muzzle velocity — be careful with your units). Divide the kinetic energy of the bullet by the total energy supplied by the powder to get the energy conversion (chemical to kinetic) efficiency of the gun. Let's take a .44 Magnum with a 200-grain (13 g) bullet and 18.3 grains (1.2 g) of powder, and a nominal muzzle velocity of 1520 fps (463 m/s), based on this table. The energy in the powder is 4.8 kJ, and the kinetic energy of the bullet is 0.7 kJ, for an efficiency of just 15%. TenOfAllTrades(talk) 15:47, 13 November 2010 (UTC)

That's a lot worse than I thought. So the wasted energy is manifested in the form of heat mostly? ScienceApe (talk) 14:28, 14 November 2010 (UTC)

Read it! Cuddlyable3 (talk) 15:28, 14 November 2010 (UTC)

Uncertainty principle vs. dark energy

The Energy-time uncertainty principle is fuzzy in the amount of power in the system, rather than the amount of energy (or mass+energy) in the system, and does not ever violate Conservation of energy. Since it is energy (or mass+energy) rather than power that contributes to Gravitation, how can fuzzy power cause Dark energy? Hcobb (talk) 15:39, 13 November 2010 (UTC)

I'm not sure I understand the assumptions behind your question. I'm not familiar with any work that suggests the uncertainty principle is somehow linked to dark energy; could you provide a bit more information about where you're coming from on this? TenOfAllTrades(talk) 15:53, 13 November 2010 (UTC)

I’m guessing part of the confusion here is a matter of glancing at the ΔE and the Δt in the uncertainty principle, and thinking "oh, ΔE and Δt give units of power". However, power is ΔE divided by Δt, whereas the uncertainty principle involves a product of ΔE and Δt.

Also, although the uncertainty principle and dark energy both involve an uncertainty as to how much energy there is, the two are unrelated. The uncertainty in energy involved in the uncertainty principle is due to the wave nature of particles. The uncertainty in energy involved in dark energy is a matter of our observatories having difficulty observing the energy due to other reasons. The uncertainty in energy involved in the uncertainty principle is negligible except when dealing with very tiny amounts of energy, and very tiny amounts of time, neither of which apply when doing cosmology. Red Act (talk) 17:58, 13 November 2010 (UTC)

I think Hcobb is talking about the idea that the dark energy is vacuum zero-point energy. The problem is that the predicted zero-point energy density is much too large (by a factor of at least 10¹²⁰ in certain units). So this is a case of quantum mechanics giving a huge (and wrong) answer.

There is actually no energy-time uncertainty principle in quantum mechanics. There probably should be, but there isn't, because time isn't an observable. Nevertheless, it seems hard to avoid the prediction of an enormous cosmological constant in any theory of quantum gravity. The reason (as best I understand it) is that virtual particles look locally just like real particles. You can't have gravity couple to one but not the other unless gravity somehow has nonlocal knowledge of the system. It's not good enough to say that the zero-point energy just isn't there; it is there in the mathematics, and you have to figure out a way to get rid of it. This is a major unsolved problem. -- BenRG (talk) 01:17, 14 November 2010 (UTC)

From above: "There is actually no energy-time uncertainty principle in quantum mechanics"

Ummm... there's a link right above you to the Energy-time uncertainty principle page here. Is the Wiki wrong, or is your objection incomplete?24.21.189.34 (talk) 17:06, 14 November 2010 (UTC)

Hmm? How is time less an observable than position? You don't deny there's a position–momentum one?

I've always interpreted the time–energy conjugacy as one way of looking at, for example, the fact that you can't have a truly monochromatic plane wave of light unless it has always existed and always will. If you have a bounds on when a photon was emitted, it comports limits on how well you can know that photon's energy. No? --Trovatore (talk) 07:57, 14 November 2010 (UTC)

Time is not an observable#Quantum mechanics. It's treated as a parameter which is known, a priori, to arbitrary precision. Quantum mechanics doesn't give you a probability distribution over times, whose standard deviation would be the Δt in ΔEΔt. In classical wave theory there is an energy-time uncertainty relation and I suppose that can be adapted to quantum mechanics somehow, but it's not exactly analogous to the famous ΔxΔp uncertainty relation. It probably should be, but it's not. This is the famous "problem of time" in quantum gravity, or one aspect of it anyway. I only mentioned this because the original question mentioned energy-time uncertainty. It may have nothing to do with what I said about vacuum energy. The vacuum energy is related to the (more general) uncertainty principle, since you can think of the vacuum as being made of a bunch of harmonic oscillators whose ground-state energy is the vacuum energy, but it may have nothing to do with an energy-time uncertainty relation specifically. -- BenRG (talk) 05:12, 15 November 2010 (UTC)

Double slit experiment / video clip

I have created a 50 second clip on Young´s double slit experiment. As a I am not a physicist, I would like knowledgeable editors to provide feedback / critique / suggestions. The size is 10MB (not suitable for dial-up lines). Thank you. --Cookatoo.ergo.ZooM (talk) 21:23, 13 November 2010 (UTC)

The Double-slit experiment article would benefit from a better illustration than this diagram. But in your video the sea of waves obscures the essential explanation that at any point on the screen, the 2 separate waves from each slit meet for the first time, and their interference occurs there. It also needs to be clarified that whether their interference is constructive or destructive depends on the difference in their path lengths, which is a function of the angle. The light source is drawn larger than necessary. It's not clear why some thin ray lines are drawn from the start before the light waves appear nor why they develop into green walls. The interference pattern is static so I don't think any purpose is served by animating the gradual buildup of the waves. Cuddlyable3 (talk) 01:21, 14 November 2010 (UTC)

The size of these videos is still a huge problem - even over cable modem it stops and starts. I believe that you should shoot for this video to be viewed at the preview size you have it at now, and make the file as small as needed for this one purpose. Therefore I think you should make the slits a little larger, or a little more prominent. I suspect it may help if the slits are made two different colors, and the waves are made two different colors, so that you can see the interference more clearly... but that too would add confusions that would have to be explained.

The video as-is already has some value, but I think that a little tinkering well more than double its usefulness. I agree with most of Cuddlyable's comments except I do think that the gradual buildup of the waves is quite useful and definitely should not be taken out. Wnt (talk) 04:41, 14 November 2010 (UTC)

1. Technical improvement: I have reduced the file size to 1.1 MB. The clip has to be modified because the resolution is now very poor. I will work through the rest to see what can be done. Thank you for the response. --Cookatoo.ergo.ZooM (talk) 09:25, 14 November 2010 (UTC)

This loads and plays very well for me, at least, and at the inline resolution the image is still very clear. I would encourage you to use this as is; any further improvements might involve adding more detailed information at the end. Wnt (talk) 15:19, 14 November 2010 (UTC)

(Referring to the modified clip). Half-way through the clip at 0:22 the light source inexplicably changes to somewhere between the slits. The illumination from the source to the slits should be shown. It is wrong to show the constructive/destructive pattern forming immediately the light leaves the slits. Cuddlyable3 (talk) 15:22, 14 November 2010 (UTC)

There are two takes in the clip: Take 1 shows the 2 separate waves (one originating at each slit) and their "intersection" (= interference). At 0:21 the 2nd take commences. This take shows the "composite" (ie single) wave as it is generated by constructive / destructive interference.
As a professional ignoramus I may easily misunderstand aspects of this experiment.
I will render take 1 with the light from the slits showing as 90° sectors (+/- 45° from the axis), rather than 180° sectors. The interference will then start a bit off the slit panel. I will also tweak take 2 so that the composite wave does not start in between the two slits but further "back". Thank you for the comments and greetings from Vienna. --Cookatoo.ergo.ZooM (talk) 16:44, 14 November 2010 (UTC)