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Law of impulseby Physicsissuef
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#1
Apr608, 04:04 PM

P: 908

Annihilation (connecting positron and electrons) gives us 2 gamma rays. I.e
[tex]^{0}_{1}e+ ^{0}_{+1}e \rightarrow 2 \gamma[/tex] In my textbook says: Why [tex]e[/tex] is written like [tex]^{0}_{1}e[/tex] instead of [tex]e^^1[/tex] or something?? 


#2
Apr608, 04:26 PM

P: 455

1. Your text must be translated from one language to another.
I think when it says "preservation of impulse", it just means conservation of momentum. 2. That clumsy notation is what is used in nuclear physics. It means nucleon number=0. The subscripts are the charge. That notation is almost never used for simple electrons. 


#3
Apr708, 12:49 AM

P: 908




#4
Apr708, 01:05 PM

P: 64

Law of impulse
Conservation of momentum means that the total momentum is the same after the reaction as before. For a particle with mass the momentum is
[tex]\textbf{p}=m\textbf{v}[/tex] where m is the mass and v the velocity of the particle. For photons: [tex]p=\frac{E}{c}[/tex] where E is the enery and c the speed of light (in vacuum). 


#5
Apr708, 02:57 PM

P: 908

in this case, it imply for the gamma ray? So
[tex] p=\frac{E}{c} [/tex] is the one that we look for, right? 


#6
Apr908, 12:53 PM

P: 64

Yes in the final state you have two phonons, but in the initial state you have one positron and one electron and they have mass.



#7
Apr908, 01:42 PM

P: 908

so mv=[itex]\frac{E}{c}[/itex], right?



#8
Apr1308, 01:00 AM

P: 908

and why p is in long brackets i.e p ?



#9
Apr1508, 06:58 AM

P: 908

is it same with conservation of energY?



#10
Apr1508, 10:32 AM

Mentor
P: 11,866




#11
Apr1508, 11:01 AM

P: 908

conservation of energy and conservation of impulse are same in this casE?



#12
Apr1508, 12:06 PM

Sci Advisor
HW Helper
P: 4,738

[tex] E^2 = p^2c^2 + m^2c^4 [/tex] So for massless particles (as the photon): [tex] P = E/c [/tex] There is no way that you can get mv = E/c Conversvation of total energy and conservation of momentum/impulse is not the same thing. 


#13
Apr1508, 03:27 PM

P: 908

E is energy, so what is the difference?



#14
Apr1508, 03:30 PM

Sci Advisor
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P: 4,738

i) momentum is a vector, energy is a scalar.
ii) you have massive particles on the Left Hand Side (LHS) of the reaction. 


#15
Apr1508, 03:39 PM

P: 908

So conservation of energy is for the gamma ray, and conservation of momentum for the particles, right?



#16
Apr1508, 03:41 PM

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P: 4,738

NO
conservation of energy for the whole reaction & consercation of momentum for the whole reaction! momentum(LHS) = momentum(RHS) & Energy(LHS) = Energy (RHS) Both equations must be fullfilled. RHS = right hand side LHS = left hand side 


#17
Apr1508, 03:46 PM

P: 908

And what is conservation of impulse, said with simpler words? Maybe some analogy?



#18
Apr1508, 03:57 PM

Sci Advisor
HW Helper
P: 4,738

impulse is just change of momentum. That is the real definition of it. So it can't be conserervation of impulse.
Pam wrote: "1. Your text must be translated from one language to another. I think when it says "preservation of impulse", it just means conservation of momentum." in pots #2 And sometimes, authors uses impulse sloppy instead of momentum. So I confirm what Pam wrote, what is meant is conservation of momentum. 


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