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Comments:
<0> nite
<1> An accelerating object is at 0.049m at .200s and 0.096m at .240s.
<1> What is the average velocity between these two points? [ive calculated this to be 1.2 m/s] At what time is this value the instantaneous velocity?
<1> the second aprt of the question is confusing to me
<1> wouldnt it just be .240s?
<2> No
<2> it's the time at which the object is going at 1.2 m/s
<3> anyone able to lend me a hand with a sort of engineering question?
<2> What kind of question Naveg?
<3> i need to find a good way to rotate something around a metal cylinder (like a soup can) with almost no friction
<3> like a track on the outside of the cylinder
<3> the whole thing has to be metal, cause its gotta conduct
<2> So you want to rotate what around a metal cylinder?
<3> like a metal rod or something similar
<3> let me put all this in context
<3> i'm making a safe that can be cracked using only knowledge of physics
<4> http://charliebrown.outwardhosting.com/~kmw/Super_Bowl_XLI.png
<3> i'm putting a radio transmitter inside the can, and giving the crackers a receiver
<3> the transmitter is broadcasting the code
<3> but the can is a faraday cage
<3> so the receiver doesnt pick anything up
<3> what they have to do is rotate a metal rod to the other side of the can, and connect it to their antenna
<3> so that the can now acts as part of their antenna
<3> rather than a faraday cage
<3> so what i'm working on now is how to rotate the rod to the other side of the cage
<2> Why does it have to rotate?
<1> purple, how can i find that out from what was given to me?
<3> well the idea is that it has to eventually connect up with their antenna, which is on the other side
<3> so that they can receive the code to the safe
<2> You have the average velocity right? Just divide the average velocity by the acceleration of the object to find the time at which the instantaneous velocity is the average velocity
<3> purplepenguin, a rotating crane with a magnet hanging down is used to rotate the rod around the can
<3> so friction has to be low enough that the magnet will bring the rod around the can when it rotates
<2> You can find the acceleration by 1/2*a*(dt)^2 = dx and plugging in the values for dx and dt
<3> i'm trying to figure out the best way to allow the rod to rotate around the can with little friction
<2> Oh,so the can and the rod are cocentric?
<3> yea i guess so
<3> the rod is attached to the can
<3> but can rotate around it
<3> it sticks out
<3> do you understand what i mean?
<3> so it has to be on a track or something
<2> is this rotation like two gears rotating?
<2> except minus the teeth, and using only friction?
<3> lets see if i have a picture i can show you
<3> one second
<3> http://img226.imageshack.us/img226/7366/safedt9.jpg
<3> so when the "metal post" rotates around the can by being dragged by the magnet, it contacts the rod on the other side of the safe
<3> which can now be connected (using tin foil) to the receiver
<2> How is the metal post attached to the track?
<3> this is what i'm trying to figure out
<3> i'm not sure how to do that
<3> instead of a track it could be like a wheel
<3> a metal wheel
<2> You want to move the magnet so that it moves the metal post
<3> yes
<3> the crane rotates around
<3> its hard to see in the diagram
<2> so the goal is to make a connection from the antenna to the metal post through into the faraday cage
<3> correct, once the faraday cage connects to the antenna, it becomes part of the antenna and doesnt block the signal
<2> You could just attach the metal post to the faraday cage with a conducting wire, couldn't you?
<2> The problem would be, how the magnet would manipulate the faraday cage + metal post system
<3> yes, but how would it rotate around the can then?
<2> I'm ***uming the metal post is a ferromagnet
<3> it can be anything that a magnet attracts and that conducts EM waves
<2> If the magnet in the post is sufficiently separated from the whole faraday cage circuit you could move it with the magnet on the crane in a rudimentary manner, ***uming that the faraday cage was free to rotate or that the metal post was on a rotating pivot
<2> I think
<3> right, exactly - either the whole cage has to rotate, or the post has to rotate around it
<3> i'm wondering how i can accomplish that
<3> like, with what parts etc
<2> you could put the whole apparatus on a rotating platform
<3> if the radius of the beam separating the post from the cage is bigger, is it easier to rotate with a magnet or harder?
<2> or you could put a thin ring of metal with a layer of nonconducing grease around the faraday cage that the metal post attaches to, that will allow it to rotate
<3> you mean conducting grease
<3> it has to conduct
<3> or it wont work
<2> I guess
<3> so like a greased up rotating metal gasket/bezel
<2> Yeah, basically
<3> will that slip down the can though?
<2> the question I'm thinking of is how to move the metal post with the magnet
<3> it has to stay in the same vertical position
<2> Not necessarily, you can put a thin rim undernet where the gasket will be so that it doesn't fall
<3> you adjust the boom of the crane so that it has the same radius of the cage/post system
<3> and rotate it
<2> is the crane above the cage?
<3> yea, its probably going to be mounted on the ceiling of it
<3> the diagram is wrong
<3> it was a quick mockup
<3> it has 360 degree rotation
<2> Okay
<3> the centre is right above the cage
<3> so since the magnet attracts the post, the post rotates with the crane
<3> sound good?
<2> Alright, I think that would work, so long as the magnet was not interfering with the faraday cage or the circuit with the antenna
<3> alright, thanks so much for your help
<2> np
<5> stupid thing...
<5> arg..
<2> Hmm
<2> why do you say the Schwarzchild radius, what do you mean by that
<5> purplepenguin: it would be the radial component in the schwartzchild geometry
<2> I wouldn't know what that is
<2> Oh, it's a relativistic star
<5> arg....not enough characters in topic...
<5> there we go...
<5> ARG!
<6> what are you trying?
<5> topic is too short :(
<6> shorten it to 'Physics. channel website: ... '
<2> Do you actually need to use GR to do this
<5> well im sure you can do it without, but you would get a "wrong" answer
<2> boo
<5> with gr your answer will be less wrong :)
<2> cl***ical mechanics is a bit easier
<5> indeed
<7> JabberWalkie: nobody got it on their first try eheh
<7> hehe*
<5> :)
<5> im working my way through this one...
<5> ive got the 4-velocity of the comet at the point, so far
<5> all i need to do is project it
<5> its quite a simple problem, mathematically....theres no integration or whatever
<5> some algebra...
<5> but it really requires a good understanding...
<7> You have to know what a Schwartzschild radius is.
<5> yep
<7> and what an impact parameter is
<8> Any good book recommendations for introductory electromagnetism courses? I've heard Griffiths' is quite good but I'm looking for more recommendations (it costs 40 quid :-/ )
<7> how 'introductory' are you talking
<7> griffiths is good but not that easy
<8> "introductory" as in "I'm competent with vector calculus, remember elementary EM from high school well but definitely nowhere near the Jackson book"
<8> (i.e. this is a 1st year physics course but we tend to go into quite a lot of detail in our courses)
<7> then griffiths is very good
<7> so long as you are ok with vector calc
<8> I've tried my luck with Duffin as well.... but, while the text is good, the problems are not that great (i.e. it doesn't have any "very easy" problems which I usually need before diving into the harder ones)
<8> and the berkley series doesn't use SI and it really gets to my nerves :(
<7> haha
<8> microacg: Ok, thanks !
<5> hmmm.....my answer is way too nice....
<6> a stationary observer... :)
<5> its magic.....
<6> must be :)
<1> An accelerating object is at 0.049m at .200s and 0.096m at .240s.
<1> What is the average velocity between these two points? [ive calculated this to be 1.2 m/s] At what time is this value the instantaneous velocity?
<5> depends on how it is accelerating
<1> in what sense
<5> well it could be anywhere inbetween
<1> basically the question is, when is the velocity 1.2 m/s
<1> right?
<1> since we want lim delta t->0
<5> well you cant tell, you can make it accereate such that that point is anywhere between t=.200 and .240
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