Wikipedia:Reference desk/Archives/Science/2008 May 24

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[edit] May 24

[edit] Concrete dampening and evaporation

Recently I witnessed some light rain which evaporated fairly quickly after landing on the concrete. However, I noticed it on two sections of the sidewalk which were of about the same color and exposed equally to the rain, and one looked much wetter, as if the rain wasn't evaporating as fast. Why might this happen? What hidden variables might be in effect? 69.111.191.122 (talk) 00:08, 24 May 2008 (UTC)

I wonder if the roughness of the surface could affect the evaporation rate, or your perception of its wetness. -- Coneslayer (talk) 01:33, 24 May 2008 (UTC)
I think the damper part of the pavement was a little rougher, so that might have been it. Otherwise, they were about the same in terms of exposure to both sunlight and rain. 69.111.191.122 (talk) 02:05, 25 May 2008 (UTC)
not to mention the possible absorbancy of the micro-surface of the concrete. Richard Avery (talk) 07:12, 24 May 2008 (UTC)
Before the rain, was the sun out and was the wet pavement in the shade then? DirkvdM (talk) 18:48, 24 May 2008 (UTC)
Also one part of the pavement might be above someone's basement or a sewer tunnel and the other have earth directly underneath. --Lisa4edit (talk) 01:48, 25 May 2008 (UTC)
Could be due to the porosity or the concrete —Preceding unsigned comment added by 79.76.141.94 (talk) 04:03, 25 May 2008 (UTC)

[edit] Studies

Are there any studies being conducted (besides this one: http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0050193&ct=1#journal-pbio-0050193-box01) that are looking for the effect of bisphenol a in humans through out the lifespan? They must be looking at only bisphenol a's effect, I don't want any other chemicals in the study being studied for their effects.68.148.164.166 (talk) 01:16, 24 May 2008 (UTC)

There are more studies on this subject than you can possibly read. Searching Google Scholar for recent articles on bisphenol toxicity in humans returns 2890 hits as of today. Hope this helps ;) --Dr Dima (talk) 03:46, 24 May 2008 (UTC)

[edit] How to deal with very recent developments? Pulser pump, mechanical mathematician, accumulating solar barbecue, dripper trackers.

I tried a couple of years ago to add the pulser pump to wikipedia. It is a simple river driven pump for 3rd world use that has no moving parts. I made one in Ireland and it is now 20 years old and still working. I have posted details on the web including video but for whatever reason, nobody in the scientific community has ever bothered to independently verify it. Its component parts are well know. (Trompes and airlift pumps). So it is a very short mental hop to realize that they actually work. But nobody has bothered verifying it. I invented the mechanical mathematician in late summer 2007 and it is now on solarcooking.org It is a tool for making parabolic dishes and moulds from clay. I invented the dripper trackers early this year and solarcooking.org put them in their recent news section. Dripper trackers are appropriate tech timer/counterweight devices for tracking solar reflectors. There are no patents involved. Is there any way to fasttrack them onto wikipedia so more people can make and use them? Brian White BC Canada Gaiatechnician (talk) 02:07, 24 May 2008 (UTC) —Preceding unsigned comment added by Gaiatechnician (talk • contribs) 02:02, 24 May 2008 (UTC)

Short answer: no.
Long answer: Wikipedia is an encyclopedia. We have articles on things other people have reported on through reliable sources. We don't publish original ideas. What you need to do is either A) get your idea published in a respected engineering journal or B) get major news sources to publish articles about your idea, probably by proposing it to governments or corporations. Wikipedia isn't the place to come to promote your concept, I'm afraid. — The Hand That Feeds You:Bite 02:36, 24 May 2008 (UTC)
Definitely, no! Wikipedia has strong policies that require that articles are about 'notable' subjects, that can be 'traced back to reputable sources' and are not 'original research'...your proposed articles fail all three tests - and if you wrote them, you'd probably fall foul of the 'no self-promotion' and 'anti-spam' guidelines too! That's mostly because we aren't going to take your word for it that these things work - we need to have 'references' to books and magazines that have independently reported on these things.
A better place to promote and discuss your ideas would be somewhere like World Changing - a Wiki designed for accumulating information on 'appropriate technology' - or Green Technologies - another Wiki which has similar goals. I'm sure there are others. Google 'Wiki Green Technology' for example. 70.116.10.189 (talk) 13:26, 24 May 2008 (UTC)

Ok then, the pump has been on the internet glossary of pumps for about 5 years. Does that count for anything? Also, since people who might use the pump often use wikipedia for reference, is there any chance that you can add a proper definition of trompe to wikipedia? They were widely used in Canada, spain, france, and the USA before hydroelctricity. In mines especially and even to operate pneumatic equipment for building alpine tunnels! Gaiatechnician (talk) 22:19, 24 May 2008 (UTC)

And with regard to your "pulser pump", have you seen our article about the well-known hydraulic ram? Two moving parts, but it sounds quite similar.
Atlant (talk) 15:44, 27 May 2008 (UTC)
Look, it isn't a ram. (The ram is also in the internet glossary of pumps).

I dont care what you call it, You can call it a combined airlift pump and trompe if you like but why not have at very minimum a definition for trompe on wikipedia? Trompes are real things that were used for over a hundred years and which are documented in engineering reference books. Pulser pump (I do not care what you want to call it) is a combined airlift pump and trompe and has no moving parts.

The difference between 2 moving parts and no moving parts should be enough to make people understand that they are different things but apparently not. It has nothing whatever to do with hydraulic rams and the tiniest bit of basic research would make that clear.

(http://members.tripod.com/~nxtwave/gaiatech/airliftreferences/trompinfo.html) is trompe info supplied to me as a result of a web request. but of course wikipedia cannot use it because it comes from my webpage and not from a book that I have. It is clear that nobody is bothered to get the info themselves. I am sorry to be annoyed but rams are absolutely nothing to do with trompes. I am done being bated by people who will not do their research. Gaiatechnician (talk) 07:20, 29 May 2008 (UTC)

Do you find that your attitude commnly leads to success? It seems not, based on what you've said here. Just "a word to the wise"...
Atlant (talk) 13:05, 29 May 2008 (UTC)

Knowelege of trompes can help riverside people in poor countrys have better lives. You can either provide them with that information or hide it from them. Feel free to concede that it is not a ram and lets make progress here. Why hide useful information away from poor people? Trompe still does not have a proper definiton in wikipedia. Why? It has been 5 days. Ample time indeed Gaiatechnician (talk) 00:01, 30 May 2008 (UTC)

[edit] WHAT IS THE HYPOTHALAMUS

Have you tried.... hypothalamus? :-) --98.217.8.46 (talk) 03:01, 24 May 2008 (UTC)
More simply, it is the part of the brain that sends the signals from your nerves to your hormones. Rockpocket 07:40, 24 May 2008 (UTC)

[edit] Earwax

What is the best way to prevent a buildup of earwax? —Preceding unsigned comment added by 79.76.238.57 (talk) 02:56, 24 May 2008 (UTC)

Clean your ear canals regularly with cotton swabs. —Keenan Pepper 03:12, 24 May 2008 (UTC)
...But do read the instructions, and don't be an idiot and puncture your ear drum like this man. —Keenan Pepper 03:16, 24 May 2008 (UTC)
I was told never to put anything in my ear that was smaller than my elbow! —Preceding unsigned comment added by 79.76.238.57 (talk) 03:20, 24 May 2008 (UTC)
Yea, that's pretty borderline medical advice there. I don't clean my ear canals with cotton swaps any more after I crammed so much ear wax that I couldn't hear out of one ear till a $300 doctor's visit (paid for by insurance, I think). I don't clean them at all any more and I haven't had any problems in 3-4 years (since the doctor visit). --Wirbelwindヴィルヴェルヴィント (talk) 03:27, 24 May 2008 (UTC)
Everything I've ever read or heard about this states pretty clearly that cotton swabs are a bad idea. They remove some of it, sure, but they also push the wax farther in and pack it in more tightly. In fact, even our own article on ear wax says, "Cotton swabs, on the other hand, push most of the earwax further into the ear canal and remove only a small portion of the top layer of wax that happens to adhere to the fibers of the swab." -- Captain Disdain (talk) 11:33, 24 May 2008 (UTC)
those cotton swabs r no good for that purpose, i have been told by an ear doctor (Otolaryngologist? ear MD?)... i personally use my shower head (where the water comes out) with low pressure and direct it in various angles at my ears... yesterday my ear doctor found nothing in my left ear (after more than a year) and in my other ear he found something that he removed with a special "shower head"... furthermore i think that apes wont do anything about their ear wax... --Homer Landskirty (talk) 04:08, 24 May 2008 (UTC)
There are a number of relatively safe dissolving preparations available over the counter from chemists. A few drops of solvent is dripped into the ear canal and the canal is then lightly plugged with cotton wool. The dissolved wax runs into the cotton wool. Do it at night and one ear at a time and you should be wax free in no time. Richard Avery (talk) 07:10, 24 May 2008 (UTC)
Any oil akin to say, soy oil, will produce similar results. Use a proper tool for the job, and inject a wee few drops before going to sleep. Use cotton to keep it in, and repeat next evening. At the end of the treatment (my doctor told me to do it three evenings in a row) some comfortably warm water should be used to rinse the ear canal. Showerhead excels here. Not too much pressure, as Homer pointed out, but enough and from many enough angles that you cover it all. You can use a cotton pin to check how much is left, a day and two after. Removal of earwax is nearly as easy as removal of snot, so I wouldn't mind the medical advice disclaimer. Scaller (talk) 08:48, 24 May 2008 (UTC)
I use 70% isopropyl or ethyl alcohol to wash off my earwax, free disinfectant too.--Lenticel (talk) 12:08, 25 May 2008 (UTC)

[edit] New portal

Hello every one! I've created a new template on molecular and cell biology. Since, I do not have much experience and confidence in editing such sophisticated material, and also don't possess the requisite expertise in the field, I request others to go through the template and edit it in accordance. Moreover, I felt this was a very fundamental topic, which didn't have any template, so created that.

I don't know if there is a provision to notify users accessing articles related to the template that such a template has been created, and that it could be included on the pertinent pages.

Regards.

KetanPanchaltalk-TO-me 07:12, 24 May 2008 (UTC)

Try Wikipedia:WikiProject Molecular and Cellular Biology - the talk page there would be the best place to contact people involved in the articles your template would be useful on. --Tango (talk) 15:06, 24 May 2008 (UTC)

[edit] Peer Reviewed?

[1]

I'm confused by the article. Are they saying that bisphenol A actually occurs naturally in the environment? Chemical reactions actually happen in the environment to creat bisphenol A with or without catalysts? And if so, is this peer reviewed?


Another entirely different question:

Does industry funding have any importance once a piece is peer reviewed?

I mean, if a piece is industry funded, and the funder decides that results are not in its intrests, they can pull out funding, or if in the middle of the research, they can decide not to pay for the rat, and then even if the piece get's peer reviewed, the peers would obvious see something wrong, and the piece would have to be revised.

If the piece is industry funded, and then it gets peer reviewed, if say the rat cage was not cleaned (and this was on purposely not cleaned, so that the results would indictate not bisphenol A release (let's just say)), the peers would find it and the piece would have to be revised anyways.

So is there any way that from where a piece is funded is important to know on top of peer reviewed statusstratification?68.148.164.166 (talk) 06:54, 24 May 2008 (UTC)68.148.164.166 (talk) 09:48, 24 May 2008 (UTC)


I don't think the implication (from reading the abstract) is that it occurs naturally. I think they are suggesting Bisphenol A is made synthetically, but then leaked into the environment where humans and other animals are exposed to it. It would appear that the greatest exposure comes from our diet.
On reading the information for authors, its not entirely clear whether review articles are peer reviewed in this journal. Some journals do peer review reviews, others do not. You could always email the editor and ask him or her.
Authors typically have to declare any conflict of interest at the end of a manuscript, where industrial funding would be declared. However, since this is a review article (rather than original research) the scope for COI is limited. The authors are simply summarising the literature, rather than reporting new findings. However, to address your concerns. If you wanted to purposely hide or lie about some information then it is unlikely reviewers would be aware of that on a one off occasion. In time your work would be discredited though, as other people in the field would not be able to repeat your experiments. It is reproduction and repetition that determines the authenticity or scientific reports in the longer term (which is why you should take conclusions of any single paper at your own risk). Cheats may prosper in science for a while, but eventually their work will be marginalized. That all said, most scientists do factor in COI when reading articles. For example, I also treat research on lung cancer that is funded by the tobacco industry with a healthy dose of skepticism. There are also conditions factored into corporate funding sometimes. The money men may have the final say on whether you can publish some data, so if your findings are not strategically favorable to them, they could stop you from publishing it. So its very much in your interest to ensure that the funders like the outcome. For this reason, a lot of scientists refuse to accept corporate funding: because it can leave you in a, shall we say, morally ambiguous situation. Rockpocket 07:37, 24 May 2008 (UTC)
The article states that the source for exposure is enviroment or food, but the bisphenol A in the enviroment and food does 'NOT occurs naturally in this sources, but is added by pollution through human products and waste.--Stone (talk) 08:38, 24 May 2008 (UTC)
Note that peer review does not guarantee anything is correct. It also does not really attempt to spot deliberate falsification. All it does is try to make sure that the article can pass a very elementary analysis by people who are versed in the same knowledge. They are just checking to make sure it isn't totally worthless, totally embarrassing to the journal it is in, or taking up space that would be better served by just about anything else. I'm not knocking peer review, though it is hardly the wonderful system that many of those who are not involved in it think it is. It's one way of handling the question of what should be published; it is also somewhat of a deliberate screen to keep out "cranks" and non-professionals. It does not, and never has, guaranteed validity, freedom from bias, or even something simple like coherency. Don't over-trust in it. --98.217.8.46 (talk) 19:52, 24 May 2008 (UTC)

[edit] automobile wheel size

I am buying a car soon and I have a choice between the base model and a higher model.

They are practically identical except that the base model has 15 inch wheels while the higher model has 16 inch wheels.

Does this mean that the base model will run at a higher rpm (engine speed) for the same on road velocity?

Does it mean that the base model (15 inch wheels) can accelerate faster because it can put a higher torque on the road due to its smaller wheel size? 122.107.199.2 (talk) 10:47, 24 May 2008 (UTC)

The "15 inch" and "16 inch" numbers are the diameters of the wheels (tire rims). The size of the wheel does not by itself determine the diameter of the tire, because the height of the tire walls are not taken into account. See the Wiki article on tire specification for more details. --71.162.249.240 (talk) 12:45, 24 May 2008 (UTC)
The larger rims have skinnier tires - which gives the car a more agressive look that's very trendy today.
Sadly, the people who go for bigger rims and skinnier tires are not physicists. Rubber (even with steel 'belts') weighs less than aluminum so the 15" wheels (having more rubber and less aluminum than the 16" kind) are typically lighter. Whilst the difference isn't all that great compared to the weight of the entire car, driving experts like to distinguish 'unsprung weight' from 'sprung weight'...the weight of things like wheels, brakes and axles that are not being isolated from the road surface by springs and shocks is much more important to performance than 'sprung weight' - the engine, transmission, body, etc. Saving even a few pounds of unsprung weight can have a big performance impact. Also, the heaviest part of the entire rim/tire assembly is further out from the hub in the case of 16" wheels - which gives them a higher moment of inertia - which makes them harder to start and stop spinning - so you actually get more accelleration and better braking from 15" wheels than 16". The only benefit (performance-wise) of 16" wheels is that the side-walls of the tires are shorter so they flex less while cornering and braking - which somewhat improves handling but produces a slightly harsher 'ride' than the 15" wheels.
One cool trick is to buy the 15" wheels - but then swap out the tires for skinnier ones and sell the old (but unused) tires on Craigslist or whatever. Now you have the 'look' of the 16" wheels but a smaller diameter. You truly get the best of both worlds AND the reduced overall diameter gives you better accelleration (although a lower top speed) - and has the effect of lowering the car a half inch - without the cost of 'lowering springs' which looks cool (if you like that kind of thing) and typically produces better handling. The only real downside is that you have less ground-clearance and (strictly speaking) your speedometer and odometer need to be recalibrated. 70.116.10.189 (talk) 13:09, 24 May 2008 (UTC)

[edit] Studies

I know there are no completed studies that are looking for the effect of bisphenol a in humans through out the lifespan, but are there and studies in the progress or are planned (besides this one: http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0050193&ct=1#journal-pbio-0050193-box01)? They must be looking at only bisphenol a's effect, I don't want any other chemical(s) in the study being studied for their effects. Thanks!68.148.164.166 (talk) 11:30, 24 May 2008 (UTC)

You just asked that, and got an answer, 7 sections up. --Tango (talk) 12:47, 24 May 2008 (UTC)

[edit] Water Clouds

Recently returning on a flight i just happened to notice that flying at 37,00oft there was on the wings what seemed to be water but it was moving more like a gel ???? At this altitude the temperature is as far as i am aware about between -80 to -100 . Surly the rain droplets would have frozen then fallen to earth??? Which technically would mean that the clouds would become too heavy and (fall from the skies)??? How is this???? —Preceding unsigned comment added by Mmm311068 (talk • contribs) 11:57, 24 May 2008 (UTC)

Water droplets won't get any heavier when they freeze - as long as it's a very small frozen droplet it will still be suspended in the air. When the droplets get too large, they will fall from the sky as rain. I don't know why the water droplets on the wings would act strangely - my guess would be an aerodynamic effect. --Tango (talk) 12:50, 24 May 2008 (UTC)
What you saw on the wings might not have been water (particularly not that high up) but rather Deicing fluid. That will get gel-like when you freeze it. --Lisa4edit (talk) 09:33, 25 May 2008 (UTC)

[edit] Dosage of a Drug

When dosages of a particular drug are calculated such as "1mg per kg" does it matter that someone may be 60% fat and another 20% fat but weigh the same?

What is really being estimated by using the weight, is it total blood volume, metabolism etc? 78.148.84.63 (talk) 12:44, 24 May 2008 (UTC)

Pharmacokinetics#Compartmental_analysis should give some hints... surely adipose people with liver damage need different dosage and sometimes even different medication... dosage is often symptom based, too... i think those "per kg body weight" numbers r just a rule of thumb and that doctors often cant even measure the weight of the patient when they "compute" the dosage (e.g. in case of an emergency on the street...)... --Homer Landskirty (talk) 13:39, 24 May 2008 (UTC)
Usually the dosage is given as "mg per kg of ideal body weight". Often this is just ignored, as there are only certain drugs for which the adjustment for obesity is crucially important. The volume of distribution for lipophilic drugs may be greatly increased in the obese. - Nunh-huh 15:59, 24 May 2008 (UTC)

[edit] Scientists

Are doctors or engineers considered as scientists? If not, what are they considered as. Clover345 (talk) 13:03, 24 May 2008 (UTC)

An engineer isn't a scientist, they're an engineer. It's a separate category. I'd say a doctor probably does count as a scientist though - they certainly are if they're involved in research. --Tango (talk) 13:11, 24 May 2008 (UTC)
The short answer to your question is that most doctors and engineers are considered to be doctors and engineers. A more detailed answer would depend greatly on the nature of the work that a particular engineer or physician did, as well as how broadly one chose to define the term 'scientist'.
If we say that a scientist – roughly speaking – is someone who is paid to employ the scientific method to answer questions about the way the world around us works, then some physicians and engineers would fit the category while others would not. While engineers often are involved in using known principles to solve specific problems, there absolutely are engineers who conduct basic research into metallurgy, corrosion processes, aerodynamics, semiconductors, etc. Many doctors use established techniques and protocols to treat patients, but at least some participate in clinical research projects. (Any large medical research institute will have at least a few 'double docs' floating around: individuals who have both MD and PhD degrees.)
It might help if you could give us some idea of why you want to put each of these professions into a 'scientist' or 'not-scientist' box. TenOfAllTrades(talk) 14:34, 24 May 2008 (UTC)
More generally, people who are researchers are often considered scientists. Doctors and engineers, as trades, are not researchers. There are people with MD degrees and all sorts of engineering degrees who are researchers, though. And sometimes they put on different hats and become doctors and engineers, practicers of a trade, as well. --98.217.8.46 (talk) 17:18, 24 May 2008 (UTC)
Engineers read; scientists write. --Sean 00:16, 27 May 2008 (UTC)
And vice-versa. Both engineers and scientists contribute to original research frequently, but the distribution of publication frequency and the (direct) applicability of their publications differ. Depending on the field, this line is often quite blurred. Sjschen (talk) 00:24, 27 May 2008 (UTC)

[edit] Metallurgy

I have to do a presentation about metallurgy, and I need a good demo to accompany the presentation. Any suggestions? --Sturgeonman (talk) 17:59, 24 May 2008 (UTC)

Perhaps a shape memory alloy demo. --hydnjo talk 22:52, 24 May 2008 (UTC)

[edit] Spin

If I was to throw an object at a certain speed and spin, what angular velocity would I need to spin it at such that it comes to a rest? I worked it out to 5v/2r, but I'm not sure whether my answer is right. During the calculation, I saud that F=ma, and so F(k)=Ma, μMg=Ma, a=μg. Is this right? —Preceding unsigned comment added by 76.69.241.74 (talk) 18:04, 24 May 2008 (UTC)

I don't understand your question. Whatever speed and spin you throw it with, it will come to rest quite shortly afterwards, but I guess the quickest way to achieve this is to throw it with zero speed and zero spin.--Shantavira|feed me 18:32, 24 May 2008 (UTC)
Yeah, sorry the question's badly worded. If I wanted to throw a ball at velocity v (say 5 m/s), what angular velocity would it need to have so that it comes to rest and doesn't roll afterwards (for example, if I spun it faster it would roll back towards me, and if I spun it slower it would continue in the same direction, just slower). —Preceding unsigned comment added by 76.69.241.74 (talk) 19:22, 24 May 2008 (UTC)
Sounds like a case where friction is causing a force to be applied to the lower edge of the ball in the direction opposite both the ball's travel and the spin. You want that force to zero out both the momentum and angular momentum at the same time. One critical factor is the mass distribution of the ball. Is it uniformly dense, or is it a hollow shell? The relationship between mass, spin, and angular momentum are different for each. (That said, someone else will have to work out the mathematics - I'm too rusty for that.) -- Tcncv (talk) 21:55, 24 May 2008 (UTC)

You should probably also take into consideration the coefficient of friction of the ball and of the surface as well as the air resistance on your ball. Bastard Soap (talk) 23:53, 24 May 2008 (UTC)

Well, I'm assuming air resistance to be zero. As per friction, I did take it into account, for friction is a necessary component for this to work. That's what F(k) is (force of kinetic friction). —Preceding unsigned comment added by 76.69.241.74 (talk) 01:20, 25 May 2008 (UTC)

Yeah, the air resistance would be negligible assuming a smooth ball (such as one of plastic) that is not light. However, air resistance would probably need to be calculated on something such as a tennis ball or other "fluffy" ball. It definitely would need to be taken into account for a light-weight, "fluffy" ball. For simplification of the situation, though, I would have assumed the OP to be negating that (which s\he is). Ζρς ι'β' ¡hábleme! 01:45, 25 May 2008 (UTC)
What if we leave the force of friction unknown for the moment, but simply assume it creates enough of an impulse while the ball is in contact with the surface to negate the ball's forward momentum? Ignoring deformation due to the impact, what will the ball's angular momentum then be as a function of its initial velocity and angular momentum? That might be an easier way to approach the problem I'd think. I'll think about that some more though. Hmm. --Prestidigitator (talk) 03:35, 25 May 2008 (UTC)
Suggestion: Think about the problem in reverse. The ball has just stopped. Immediately prior to stopping, the a force due to friction was being applied to the bottom surface of the ball in the opposite direction of its motion (and backspin). Any such force would (I believe) affect both momentum and angular momentum in some proportion. That proportion would be independent of the amount of force involved. Calculate the relationship between that force and changes to both momentum and angular momentum, and then combine to eliminate force, and you should have the answer. -- Tcncv (talk) 04:31, 25 May 2008 (UTC)
Well, as I said earlier, I already got an answer (5v/2r). I just wanted to know if it was right. —Preceding unsigned comment added by 76.69.241.74 (talk) 17:29, 25 May 2008 (UTC)
I'm surprised that the answer doesn't depend on the coefficient of friction. A completely smooth ball won't stop regardless of the amount of spin you give it and I would expect the required spin to vary continuously with friction (this expectation could well be wrong), which would require it to either be always impossible (doesn't seem right to me) or be non-constant. --Tango (talk) 19:28, 25 May 2008 (UTC)
My calulations involved cancelling the coefficient of friction, such I guess it's assumed to be non-zero (0/0 is indeterminant). If I'm right, friction only affects the time it takes (t=v/μg).
Well, here goes. Analyzing the angular momentum about the center (of mass of) the ball:
Δp = − p0 = − mv0
ΔL = − L0 = − Iw0 = rΔp = − mrv0
w_0 = \frac{m r v_0}{I}
Using a list of moments of inertia, for a solid ball:
I = \frac{2 m r^2}{5}
w_0 = \frac{m r v_0}{2 m r^2 / 5} = \frac{5 v_0}{2 r}
And for a hollow ball:
I = \frac{2 m r^2}{3}
w_0 = \frac{m r v_0}{2 m r^2 / 3} = \frac{3 v_0}{2 r}
which seems to agree with yours, if you were talking about a solid ball. --Prestidigitator (talk) 05:55, 26 May 2008 (UTC)
I think it's only a coincidence that we got the same answer this way. When using the equation ΔL=rmΔv, the v refers to the tangential velocity of the ball, which is only equal to its actual velocity when the ball isn't slipping.
I got an answer using the following method:
Στ=Iα, F(k)R=Iα, α=F(k)R/I.
ΣF=ma, F(k)=ma, a=F(k)/m.
Then, using the equations of motion, I solved ω=mRv/I. —Preceding unsigned comment added by 76.69.241.74 (talk) 01:15, 27 May 2008 (UTC)
It had nothing to do with slipping or not slipping. I used ΔL = rΔp = − rmv0 because you stated the ball was to stop both its linear and rotational motion. You did exactly the same when you stated FkR = Iα. --Prestidigitator (talk) 00:21, 29 May 2008 (UTC)