Physics

Dear scienceforum.net, I once tried to include myself in a discussion topic that went under the label of "what is time?" in which many individuals entered with various perspectives either championing the non-existence of time with respect to change (including me) and others who took rather standard interpretations of relativity (special or general) to describe what they mean. I felt that given some of the resources or knowledge i've attained perhaps the discussion could actually go somewhere or be somewhat more productive. Over the course of those four years I had realized that philosophy had already been discussing this with already predefined terminology whi…

I'm having a discussion with my roommate. His opinion is that only lead will block x-rays. I think anything will block x-rays, if it's dense enough or thick enough. So I'm sure that aluminum foil will block the x-rays used to x-ray packages going through the Post Office. So I guess the question is "is it possible to wrap unexposed film in enough layers of aluminum foil to prevent exposure of the film by the x-rays, and if so how many layers?" Thanks.

I am looking to build a "fire place" of sorts. I need to use a solid material around the heating unit/fire that can absorb heat and slowly radiate it for a long period of time. I know there are masonry heaters that does this well. They mostly use refactory bricks (fire bricks). Is there any other material that would be even better? Any particular stone, or stone composite that would be better? What about the possibility of manufacturing a stone composit with better properties than those already in ecistence?

We are taught that calculus is all about "instantaneous changes" between 2 points, x and delta x in respect to time with x = 0 and delta x =1 For example, when we set x = 0, "are we "manipulating" time , ie stopping at a certain location " going a certain distance" as to derive the instantaneous change at that point " location" in time? If that's the case for our x = 0 "reference point" then what's the point of calculus and its "undefined location?" Another example, if x = 0 and delta x = 1, " what's the point of coordinate vectors?" example x = 3 y = 9?? Another good example, if Cartesian Space references 0 in the coordinate system -2, -3, -1, 0, 1 , …

Sixty Celsius is about the maximum you can handle without burning your hands... That piece of metal left under the sun, or a car bumper, or the beach sand can burn your skin but the ambient temperature is only 30 Celsius. What is going on ? How does the metal gets way hotter ? It is not only the thermal conductivity from the material-to-skin; it is truly hotter. Or seems to be

Here's one that has been bugging me for a while now: In my tiny country (that nice little swamp at the end of the river Rhine) the winter months are crap. Usually above zero degrees (Celcius) and wet. The air humidity is generally at 90% or so. Why does wet air feel so cold? In fact, it feels less cold when it is -1 degrees, because the air is generally a lot dryer... is it me or is that tiny fraction of water really responsible for a major difference in heat transfer? At 5 degrees the vapor pressure of water is about 10 mbar, so the volume (or molar) fraction of water in air is <1%. How can it have an effect that I can really feel? The Cp value o…

Hi. Is there a formula that can calculate the resultant temperature of water when two different amounts with different temperatures are mixed? e.g. 100ml @ 20°C added to 50ml @ 60°C = 150ml @ ??? °C Joe

I've been researching for a while now but i can't find anything that answers my question. I'd like to know what are the radiopaque materials? That is, which materials can x-rays not pass through? Thanks in advance.

A valuable thought experiment from neighboring thread - how would physics develop if there would be no Einstein, especially regarding gravity? While modern physicists might say that without him there would be "weeping and the gnashing of teeth", his introduction of general relativity a century ago seems to be event of extremely low probability (?) - so what if it wouldn't happen? Or imagine some hypothetical other civilization e.g. SETI is looking for - would it automatically get to GR? If not, how their physics would develop? Beside Newton's gravity, there was also known GEM before ( https://en.wikipedia.org/wiki/Gravitoelectromagnetism ) - just tak…

A few days ago I started reading Ira Flatow's Present At The Future (On a recommendation), and in the introduction it discusses his misconceptions about why an airplane wing keeps a plane in the air. On the Bernoulli's Principle 'error': Apparently this is a very common error made by physics teachers across America. He says that: I'm not entirely sure this is true. The book doesn't give any math; just this explanation. One of my questions is: How does an airplane's wing make air go down? It doesn't really make sense to me. I have with me a physics textbook I permanantly 'borrowed' from my 8th grade science teacher, and in it it says: It seems that my textboo…

What is the resistance of a capacitor? From the data of experiement, I was convinced that the resistance tends to zero. Is this the truth? There's a part of empty space between two parallel plates. Isn't the resistance very large as it seems it's hard for electrons to pass through these spaces. How do you reconcile the two facts?

I've been poking about on the internet again (as you do) and found a whole load of stuff by creationists about the problems with carbon 14 radiometric dating. Specifically they report (with some glee) that coal has been found to contain measurable amounts of carbon14 which it should not of course because it is about 300 million years old and dates from the carboniferous period. C14 has a half life of 5730 years and is only good to date objects to 50,000 years or so. Although I can find any number of references to this seemingly vital finding on the creationist sites, I can find almost no attempt to refute or explain this anomaly on serious science sites. This loo…

Hello reader, According to general relativity mass curves space-time giving the effect of gravity... I have a challenge for you. Think of (a) reason(s) why mass causes space-time to curve. I have tried...You people are more qualified than I. So you should try too. There is no prize. Why am I making this thread? ...Because I haven't seen anyone attempt to answer this question of why. If you think of a reason you like I encourage you to make our own thread in the speculations section. I don't like equations...Try to not use them. You can add them to your post in the speculations section later. ...And the challenge begins in...1...2...3...Now...Enjoy! I will …

Say a man was to fall out of an airplane from high alt (say 7km) (assuming no parachute!), what shape should he mold himself into to maximize his chances of survival on impact and how should he attempt to adjust his position once entered into the water. From what I read around in bits and pieces I got an understanding that once you reach the maximum velocity of free fall hitting the water would be similar to hitting the concrete due to its incompressibility, thus fatal in most instances. My question is, if you can adjust your shape/position so that you pierce the surface of the water on impact and survive it, what position would this be? Legs down first or? Al…

Dear all, I am a design student, studying in South Africa and need help on a current project. I am designing a barbecue that doubles as a small fireplace. the idea is that once the fire is out the fireplace can be taken indoors to warm the house. So far my design can be simply put as 2 metal cylinders one inside the other - now I want to fill the gap with a material that has a high heat retention. the idea being that when the fire is burning (cooking food) the heat will be absorbed by this material, then once the fire is out the material will start to radiate the heat. I have had a few suggestions from my lecturers including, sand, brick, stone, salt (NaCl) a…

Does the Earth have a natural frequency? Do the Laws of the Universe change with Time? How do the Laws of the Universe sustain themselves?

is dark matter nothing more than a wildcard thrown in to a mathematical equation to make the equation work? what evidence do we have for it other than that? will we ever be able to see it? what do you ( or scientists) think it is exacly?

Hi, This is my FIRST year taking physics in high school (I'm senior) and our first lab which we get to work in class is building paper tower. I'm given one paper (about 28 cm x 21.5 cm; regular A4 paper) and limited amount of tape (I do not know exact but it's not "that" much) and scissors. I'm supposed to build a tower that's 1m high and standing. I've tried many methods and still am trying. But I'm stuck mainly of two reasons: 1. I can't build a solid base 2. I can't roll papers into same sized tubes (Yeah, it's not easy rolling papers... :-X). Help is greatly appreciated!

Ever since learning to integrate in high school Calculus, I've been curious as the the physical meaning of the integral of position w/ respect to time. Over the years, the topic has come up again and again in my mind, and have finally decided to put an end to my pondering; or at least let others ponder with me. Is there any firm grasp of the concept of the meter-second already in existence? Is there a good use for this? I have come to call the integral of position wrt time "longevity" as that seems to be the best term in my mind to describe it. From what I have been able to determine with some simple thinking is that longevity can be related to amount of energy or…

or is voltage inversely proportional to current? (i've been given two conflicting pictures, one from the net; one from the television) the one from the net; voltage is inversely proportional to current. the uk uses 240 v mains, the us uses 110v mains. the us is therefore safer, but requires more current (and therefore larger cables which are more expensive) the one from television (bbc learning zone); voltage is directly proportional to current; the bigger the push, the bigger the flow i'm guessing myself that the area of the cross section of the circuit wire is key to the magnitude of the current, no? (and the voltage is key to the VELOCITY of t…

I was watching this movie where this guy fell two stories, landed on his head in the pavement, and instantly died. How realistic is this? It seems like someone would need to fall much further.

Hello. This is my first post. I am nobody with an average intelligence. I have a very basic understating of time. I want to know if this makes sense. I think time is the movement of things. I am not an expert in physics so by 'things' I mean any fundamental particles. On neutron stars, the gravity is very high. This strong gravity pulls things down, so things move slowly than, say, on the Earth. Thus, time on neutron stars becomes slow. Time on an Earth-orbiting space station goes a little bit faster, because the gravity is a little bit lower than on the surface of the Earth. Time before the Big Bang did not exist, because there were no things before the Big Bang. Whe…

I just recently finished reading the last Eragon book, Inheritance, which I highly recommend everyone read, because it's awesome. Anyway, an interesting thing in the book was a sword made of diamond that was sosharp that it could cut through virtually anything. it was described as being 'the archetype of an inclined plane'. What exactly that means, I'm not sure, but I'm guessing it means that the thinnest point on the edge of the blade is only one carbon atom thick, and gets progressively thicker to form a perfect slant right down to the atomic level. Now, obviously there is no way we could (or would) make such a blade with current technology, but I couldn't help but …

I'm in need of a table or formula (empirical fit) of the conversion of ethanol/water mixtures weight percentage to volume percentage, and back. I know how to calculate the theoretical conversion (ideal mixtures), but I doubt that ethanol/water can be considered ideal. I also found an online conversion tool, but I'd really prefer to have all data neatly in a table. Does anyone have a reference for me? p.s. If the volume percentage is as simple as taking the volume of the pure ethanol and the volume of the pure water, and then dividing the ethanol volume by the sum of the water and ethanol, then I am just a nut. (My results for that don't seem to correspond with th…

Is or does the source of ALL have intelligence? Were the sources of energy or matter just coincidental?