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Lots of general questions

  1. Dec 1, 2007 #1
    I have always had a lot of simple questions that i cannot search on the net, so i stumbled upon this community where there are so many smart people, so i would appreciate if u guys can answer my questions which i had for many years

    from young i have always wondered why i get an eerie feeling when metal gets scratched or when a fork scratches a porcelain plate, can u guys please tell me why?

    many a times, old folks tell me not to let a fan blow at you for a long time, they say we may get a condition, is it true

    will locking our joints cause damage to them (e.g. standing with locked knees)

    is it possible to transfer electricity wirelessly (i know transformers can, any other way)

    is the sun rotating?

    whats the best energy conversion rate of the best solar cells

    why we can only perform nuclear fusion and fission on certain heavy radioactive elements but not on other elements

    are the black holes billions of light years away actually exerting a sucking effect on us

    does the radiation from phones actually hurt us?

    is it true we can never determine our true speed because we do not know what is the relative stationary point in the universe

    does the universe has a center of mass

    if the universe is expanding, is it expanding from a point of origin, if yes, where is this point
  2. jcsd
  3. Dec 1, 2007 #2


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    I'll try and answer a few.

    Yes, the sun is rotating. I think Galileo was the first to notice this by observing sunspots.
    You should firstly note that black holes do not "suck." In general relativity, massive bodies curve spacetime which gives the gravitational field. Thus a black hole has a gravitational field around it whose strength depends upon its mass. Since your question asks about a black hole billions of light years away, I would say that the curvature of the spacetime around us is far more greatly influenced by bodies that are in viscinity, and that any effect from a body billions of light years away would be negligible.
    Your speed depends upon the reference frame in wihch it is measured. It depends on what you mean by the "true speed."
    No, I don't think so.
    No, there is no origin from which the universe is expanding. This would break the cosmological principle, since it would suppose a special place in the universe.
  4. Dec 1, 2007 #3
    I've got a fortran program to debug atm... so needless to say this thread now has my undivided attention :biggrin:

    It's not a very nice sound! No real physical reason for that.
    No. People used to think you caught a cold from actually being cold. The reason for it was that in cold weather people stay indoors so the pathogens that cause disease spread faster. (That's as best I understand it, but biology isn't my forte!)
    Again, biology... but probably not. I have heard this discussion come up in the context of martial arts, and as I best I know there is no common consensus on whether or not locking your joints when you throw a strike damages the joint or not. But standing with locked knees I'm inclined to doubt would do you any harm.
    Yes. It's been known it's possible to do this since the 1830s. See here.
    A quick google suggests about 18%.
    Actually, this isn't true. We can only perform fission with very heavy elements, and only perform fusion with very light elements.
    The stability of the nucleus of the atom is a balancing act between the strong nuclear force (which pulls nucleons together) and the Coulomb interaction (which pushes the like-charged protons apart). Large nuclei are straining under the force of so much charge packed into one place, so they are relatively easy to split apart. At the other end of the scale, the overall stability of one helium nucleus and a free neutron is greater than that of one deuterium and one tritium nucleus separately, so if you can get the two nuclei close enough together then that reaction occurs. The difficult part is getting them close enough for the extremely short effective range of the strong force to take hold (working against the coulomb interaction).
    I think there is still a certain degree of controversy... but if a consensus exits, I'm pretty sure it's that they don't.
    Further to what christo said: We believe that there is no such centre against which all motion could be considered. If there was, in an expanding universe it would probably have to be at the geometric centre, which we don't believe exists (which sounds like a really wierd idea...spacetime is fun :biggrin:). So given that there is no such thing as "absolute" motion, (which is what I think you meant by "true"?) all we can do is discuss how quickly different observers measure you to be travelling.
  5. Dec 3, 2007 #4


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    Here's one idea: http://amasci.com/amateur/screet.html
    Intel also at a show, presented a subnotebook that had wireless charging capabilities.
    How did he know the sunspots weren't the ones moving independent of the entire body?
  6. Dec 3, 2007 #5
    No Idea. But dissonances in music create an unpleasant sensation. Perhaps it is related. As opposed to the "eerie" feeling one gets hearing a 14 cycle note which involves sound transmission by bone instead of ear which causes an understandable auric dissorientation when one does not understand the source of the feeling.

    Not that I am aware of. Check a medical forum, but it sounds a lot like the old wives tale of not being in a draft to avoid a cold.

    No. At least not in the short term. Standing still for ten days is likely to cause damage, just like running for the same length of time is likely to cause damage. Locking a joint requires both sets of muscles to maintain equilibrium in a "locked" position. Short of long term or extreme (say...siezure for example) muscular equilibrium this seems unlikely to be permanently damaging, or muscle relaxants would work better.

    Absolutely! Think radio. ( Not that fraud Marconi, but Faraday )

    Yep. As a previous poster noted with regards to Gallileo...it still moves.

    No idea what you mean. Do you mean theoretical exposure to collected energy, or storing it? The former depends on the mechanism and substrate, the latter on a whole bunch of variables.

    We can do either on any of them. The question is whether it is worthwhile. The quantum energy curve shows us that, as an example, fusing hydrogen is really a good way to release energy. Fissioning ( breaking ) U238 is also good. If I am not mistaken it is in the top of the curve say around Carbon, that the energy requirements to break, or fuse a bond is so high there is no reason to do it, not that it can't be done.

    No more, and no less than any other celestial body of that mass at that phenomenal distance. Put it this way: A friend throws a cricket over your shoulder. The cricket in free flight creates a million times more "sucking" motion on your body than the biggest black hole you can find.

    The jury is still out on this one. Some early studies say yes, others no. Stay tuned.

    Umm...no? But why? Relativity aside, how would knowing how fast one point on the surface of a balloon which is expanding be usefull to any other point on the surface of the balloon? Unless you think the Universe might be expanding unequally in different directions, which is a whole different kettle of fish.

    Of course, just look for the big bang. But unless one is looking at exceedingly large phenomena it is not usefull. Like knowing that all things on, or very near, the Earth react the same because they react as if they had the same center of mass as the centre of the Earth but when in very close proximity they react slightly differntly. Unless you could introduce a mass outside the Universe and ask how it relates it is kinda weird. Things inside the Universe will react with things near them, much more than the "centre" mostly because of: F=K*(M1*M2/D) and at Universe distances D gets Really, Really big making F almost zero.

    If I am not mistaken Black Body Radiation says look in the direction of Deneb, but check say a NASA website. Scientists look back into time to the early creation of the Universe by peering into deep space to see what happened in the past because the longer it took the light of the image to reach us, the farther back in time it must have originated from.
  7. Dec 4, 2007 #6
    but thats mine
    being out in the cold for a short amount of time won't do anything to you
    yes technically you would be weaker by just a little bit but not enought to care about

    for more then just a little bit you could get hypothermia

    just so you know all of these things i am saying is when it is like snowing outside
    not by just having the fan turned on
  8. Dec 4, 2007 #7


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    When I was at Plebe summer at the Naval Academy, we didn't have air conditioning and Plebes would point their fan on them while sleeping in a vain attempt to be cool. They generally woke up with a condition they liked to call "plebe hack" - a mild cough, but no real sickness. Not sure exactly what caused it or if there really was a corellation as opposed to jus a percieved one.
  9. Dec 5, 2007 #8


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    I've seen that attributed to the fan blades ionizing the air, but I've never really bought into the whole ionization thing fully. Personally, I suspect that since fans obviously disturb the air, they tend to stir up more dust and direct it into one's face than would normally be aspirated from calm air.
  10. Dec 8, 2007 #9

    thanks to all of you for the replies, now, at least most of my doubts are cleared
    i really appreaciated all u guys help, even though some were not related to physics, u guys still threw in ur knowledge, thanks
  11. Dec 8, 2007 #10


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    You're more than welcome. Now, stick around for a few years and contribute your own knowledge to those seeking it. There will always be someone out there looking for something that you might have the answer to.
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