insane_alien

well, it is actually the neutrons that are controlled. the reactor exists in a slightly super critical state normally. so if there were no controls it would heat up and eventually melt down. so it needs to be controlled. the neutrons emitted from a fission are fast neutrons which are crap for starting more fission reactions. so first they need to be slowed down by a moderator like graphite. then, you need to limit the number of them so you can control the rate , or if need be, shut it down completely. this is done using boron and (cadmium?) rods. these absorb neutrons without releasing more. since you take neutrons out, the reaction slows down. and this is all balanced nicely to keep a constant output.

farsight, then explain how i can hold up my arms 10 seconds apart. anyone can do it. put your left hand up then down, count off 10 seconds and then repeat with your other arm.

there is always pencil and paper. as many digits as required.

hotcommodity, the weight bearing steel structure was supporting thousands of tons. this provides some pretty large forces. now, steel has a limited strength. you can for example sit 3 billion kg of steel on your standard I-beam and expect it to hold. the forces in the WTC were a lot less but i was going for overkill to make the point, steel is not some magic, indestructable substance. the point is there is a load limit to steel structures. WTC was over engineered, there was a lot of redundancy in its structure. this still had a limit. a fair amount of the supporting structure was taken out by the impact but it was nothing too serious, the buildings could still stand safely, but were still compromised and under some stresses that weren't usually there. so, we have a steel structure that has had its weight bearing capacity severely reduced and its on fire. not just any fire, its aviation fuel so its hotter than if just some papers had caught on fire and burned for a bit. now, heat reduces the load bearing capacity of steel. a 25*C Ibeam will support more than a 1000*C I-beam. The steel didn't heat up to some magic temperature where all of its load bearing capacity disappeared, it was gradually decreased until some key components failed and then the structure pancaked for the rest. pancaking, as it happens, would produce a nice neat collapse unless on the way down it encountered one side was significantly stronger than the other which would cause it to spill over. we're talking 1.5-3 times stronger here. if that was the case then the engineer who built it should be shot for either extreme negilence(weakening one side) or bad design(making one side stronger than the other with no good reason). but, that didn't happen. it was neat. its an example of a good piece of engineering failing well.

any effect that is present would likely just be a branch of the placebo effect. if people think it will make a difference. i'm not convinced there is an effect though. i didn't see anything relating to whether the patients knew they were being prayed for or not.

i'm still not comfortable with yourdad being within 100 miles of anything with the word 'nuclear' in its name.

ummm, i can't remember. funky looking little thingys anyway... umm i gave it up cos it was kinda boring(and expensive)

i played it a few times out of morbid curiosity.

building7, wasn't that the one that had a big chunk taken out of it and smoke pouring out one whole side. it looked pretty damn damaged to me before it went down.

you could try some of my mums mashed potatoes, so dry that 1 gram can easily absorb a bathtub of water.

oh yeah, i forgot about up and down, oops. i'm ever so slightly hungover today. not quite on the ball.

nope, net force of gravity from the ring on the inside is zero, at all places. it all cancels out. its the same principle as why the electric field inside a charged conductor is zero.

live at an altitude of 100 meters

i just mentioned spheres because i thought it would come up eventually.

while wikipedia isn't a technical source by any means, the references are valid. Go look at those and they'll say the same thing.

the ring wouldn't affect anything inside it. only the outside would be affected by the rings presence.

you would need ti exhert a force of 200N to keep it there. if your not then its going to fall down.

if its a ring, it can be spun so that its orbiting the earth and it'll sit there quite nicely. hollow spheres that size have all sorts of stresses on them when they are spun in a similar manner. if its not spinning, it would need to be quite thick to support its own weight. we don't have the technology or materials yet to do this. they wouldn't be stable due to outside forces and the non-uniformity of earth. and the moon ywould probably throw it off whack as well.

theres nothing really new on it. although, they have been synthesised through chemical processes.there was something in 'new scientist' a while back about it. personally, i think they are just molecules with some interesting biological effects. then again, i define biology as 'the study of carbon compunds that wiggle'

so when you scale up the analogy so it includes the three dimensional space instead of a two dimensional space, how many dimensions do you have? (hint: 3+1) no, he's not. i've looked at the threads and edtharan is kicking your ass with logic. something you don't seem to follow 90% of the time. how exactly does a motor boat going through molases compare to light travelling through space? i mean, ok it might apply if you are discussing refraction but that requires a change in medium not a change in gravity. yes, it changes the direction through the warping of spacetime. warping of spacetime does occur, gravity probe B proved this (well, one aspect of warping anyway which reqires there to be a spacetime to warp in the first place) please don't. i actually want to see a time where humanity will be able to travel to other star systems.

dammit, how come its always other people who think up the really short, non rambling way of putting it. is there like a class on it somewhere that i could take?

The example of a truck hitting a cyclist, that cannot be used as an example of perpendicular force. it doesn't stay perpendicular. and there was only one force in the example i gave.

thats not really a good case, see, the force will only be perpendicular for a tiny tiny fraction of a second and then its parallel. a better example is a ball on a string and spinning it round, you can spin it at a constant speed but you still need to provide a force to keep it going in a circle. if you release this force(let go of the string) it flies away. this is an example of providing a force without a resulting change in speed. the velocity however changes direction all the time which is what the force is causing. this is an acceleration.

why would you need to redefine the basic definitions in kinematics to describe why a force perpendicular to motion will produce no change in speed. it will change its velocity as that is a vector. but it only changes direction, not magnitude(the magnitude is speed). this is not hard to understand. infact, i taught it to some kids in 3rd year (13year olds) physics when i helped out in some physics classes. they actually understood it, even though before that class they had never even heard of a vector. it takes force to change direction and since momentum is m*v this is also a vector quantity so it takes force to change the direction of momentum. m is a scalar quantity so its not the important part. as long as the force is perpendiculat to motion there will be no change in speed. even though there is an acceleration.

no problem. if you want to experiment on this, take a block of lead (about baseball sized) and a sledgehammer. hit the lead with the sledgehammer as hard as you can then touch the lead before it cools down. it should be quite warm.