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Comments:
<0> it was fun though...
<0> the master reminds me of some guy from street fighter...
<1> you learned how to fall properly?
<1> that is the first thing you learn in aikido
<0> nah, i didn't learn much..
<1> http://www.finjan.com/Pressrelease.aspx?id=1261&PressLan=1230&lan=3
<2> Manyfold: So it's the group velocity, but how do I find dw/dk?
<1> easy enough
<2> If w and k are constants... then the group velocity is zero?
<0> no its w/k man
<1> the group velocity is given by dw/dk
<0> w/k=c
<0> n stuff
<0> or are you not doing e&m>
<0> ?
<1> the wave equation is given by u_tt=c^2 u_xx
<0> k is the wave number and w is the freq
<1> plug your equatation into that
<1> will get you w=ck
<3> "The combined ages of a wife and her husband add up to 98. He is twice as old as she was when he was the age she is today. What are their ages?"
<1> so c=w/k
<2> But I tried v_x=v_p... where v_p=w/k... and it said I was incorrect.
<2> I'm very much not liking MasteringPhysics.
<1> what said you are incorrect?
<2> The online homework thing... MasteringPhysics.
<1> http://www.masteringphysics.com/ that one?
<2> Yes.
<2> And since this is multiple choice... of 4 questions, I missed 33% on that part of the problem. :/
<1> apollo: gime me link to the problem
<2> Hold on, I'll get a printscreen.
<2> I don't think I can give links to my problem.
<1> JabberWalkie: the acceleration an instant after the collision is g?
<0> Manyfold: no
<1> no?
<0> Manyfold: and msg me
<2> http://img258.imageshack.us/my.php?image=mphw0xj.png
<2> Part C is the one I'm on...
<2> It shows the answer I tried.
<2> Manyfold: Any clue on why v_x isn't v_p?
<1> no
<2> :/
<2> If you had to pick one out of the other three, what would you choose?
<0> 4
<2> But doesn't v_x have to be non-zero?
<0> oh i just typed a number
<0> it was quite random
<0> oh and its correct too :)
<0> what a lucky guess
<2> It is?
<0> yeah of course, the string dosn't move anywhere in the x direction
<0> just moves up and down
<2> I thought about that...
<2> But doesn't the string have to compress when it moves up and down?
<0> no part of the string actully moves to the x direction
<2> Like a string of fixed length.
<0> apollo: nahhh, its an infinitely long and perfect string
<2> Oh. :)
<2> I'll try it. Nothing to lose.
<0> like all strings...
<1> yeah i overread this i always thought about the velocity of the wave :(
<2> Thanks.
<1> JabberWalkie: you wantthe proof that 2 hermitian matrices commute?
<0> no, i want the proof that give that they commute, there exists a basis such that they are both diagonal
<1> lemma 2 linear hermitian operators A and B commute iff they have a common base of eigenstates
<0> yea, im trying to show that
<1> let A and B have the same eigenstates
<0> we can do that?
<1> A|phi_n>=a_n|phi_n> and B|phi_n>=b_n|phi_n>
<2> Thanks to everyone for the help.
<1> let |psi> be an abitrary state
<2> I was simply confusing the velocity of the wave with the velocity of the string, it seems.
<0> Manyfold: yeah, but how can we say that A and B have the same eigenstates?
<0> dont we need to proove that?
<1> |psi>=sum_n |phi_n><phi_n|psi>
<0> yeah
<1> don't we already know that the eigenstates create a complete orthogonal system?
<2> And now I'm going to bed. G'night, all.
<0> yeah, i suppose we can know that
<1> i can prove it hang on
<0> nah, i think its ok
<1> A|a_i>=a_i|a_i>
<0> yeah
<1> for i=/=j we have <a_i|A|a_j> = a_j<a_i|a_j>=(<a_j|A|a_i>)^*
<1> =a_i^* <a_j|a_i>=a_i<a-i|a_j>
<1> we are using here that the eigenvalues are real
<4>
<1> JabberWalkie: got this so far?
<0> yeah so a_j<a_i|a_j>=a_i<a_j|a_i> yes?
<1> right
<0> k
<1> so (a_i-a_j)<a_i|a_j>=0 which means <a_i|a_j> =0 clear?
<0> yeah
<1> which shows the eigenstates are orthogonal
<0> ok
<1> well the problem of showing that they form indeed a complete basis is a little beyond me atm
<0> dont worry about it
<0> lets just say we know they form a complete set
<1> so we say simply they do form a basis
<1> so we had |psi>=sum_i |phi_n><phi_n|psi> right?
<0> yeah
<1> which implies AB|psi> =sum_n Ab_n|phi_n><phi_n|psi> =a_nB|phi_n><phi_n|psi>=a_nb_n
<1> which implies AB|psi> =sum_n Ab_n|phi_n><phi_n|psi> =sum_n a_nB|phi_n><phi_n|psi>=sum_n a_nb_n|phi_n><phi_n|psi>
<1> now a_n and b_n commute
<1> problem solved
<1> JabberWalkie: do you agree?
<0> gimme a bit
<0> sum_n Ab_n|phi_n><phi_n|psi> =a_nB|phi_n><phi_n|psi> how did you do that?....what happened to the sum?
<1> AB|psi> =sum_n Ab_n|phi_n><phi_n|psi> =sum_n a_nB|phi_n><phi_n|psi>=sum_n a_nb_n|phi_n><phi_n|psi>
<1> reading that in ascii hurts my eyes
<1> :)
<3> lol
<0> ok well that does one direction
<0> what about the other?
<1> JabberWalkie: unfortunatly i have to take a shower now
<1> :)
<0> pshh
<1> but at least you have one direction now
<0> yea, but i already had that direction :(
<1> :(
<5> hi all
<6> hi! how can I measure the dynamic friction coefficient?
<7> you mean coef of rolling friction?
<7> roll a ball down an incline, measure incline, start time, end time, distance travelled without slipping etc and you should be able to get the same
<5> lets say I have a circular ventilator of 250mm diameter moving 3000 m^3 of air per hour across copper piping with water at 2-3 C
<5> how the heck can i figure out how many watts of thermal energy its pulling out of the air
<5> the room is 25 C
<7> if the temp remains constant
<7> it simply is difference is difference in calorific values
<5> so we just ***ume the air leaves at 2-3 C
<7> well not so but they havent given the temp of air after it leaves, have they?
<5> no, thats ultimately what im getting at
<7> :-S
<5> (they is me btw, im not some kid cheating on homework :)
<7> :) what isnt homework :P ?
<5> 'they'
<6> zaphybeeble thats the static coefficient.. but i need the dynamic coeeficcient.. when the m*** is moving
<5> lol
<5> ok let me put it this way
<6> there is a mu_static and a mu_dynamic
<7> once the ball is rolling there is no static coef webito
<5> i have a kg of water at 5 c and 50 m3 of air at 25c
<5> i can figure that out :)
<7> if the end temp of water is 5 then you can
<7> else u cant
<5> yeah, im keeping it static
<5> nice and linear
<5> how i like it
<7> webito say i have an incline plane, and i keep incling it till the rolling motion starts
<7> ok?
<5> actually
<7> so now i know that the component of force along the plane always is more than static friction?
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