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Law of impulse

  1. Apr 6, 2008 #1
    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:
    Now my question, what is law of preservation of impulse??
    Why [tex]e[/tex] is written like [tex]^{0}_{-1}e[/tex] instead of [tex]e^-^1[/tex] or something??
     
  2. jcsd
  3. Apr 6, 2008 #2

    pam

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    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.
     
  4. Apr 7, 2008 #3
    Yes, I mean conservation of momentum... What is that?
     
  5. Apr 7, 2008 #4
    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).
     
  6. Apr 7, 2008 #5
    in this case, it imply for the gamma ray? So
    [tex]
    |p|=\frac{E}{c}
    [/tex] is the one that we look for, right?
     
  7. Apr 9, 2008 #6
    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.
     
  8. Apr 9, 2008 #7
    so mv=[itex]\frac{E}{c}[/itex], right?
     
  9. Apr 13, 2008 #8
    and why p is in long brackets i.e |p| ?
     
  10. Apr 15, 2008 #9
    is it same with conservation of energY?
     
  11. Apr 15, 2008 #10

    jtbell

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    Staff: Mentor

    Momentum is a vector. It has both magnitude and direction. The vertical bars indicate that we are talking about the magnitude only.
     
  12. Apr 15, 2008 #11
    conservation of energy and conservation of impulse are same in this casE?
     
  13. Apr 15, 2008 #12

    malawi_glenn

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    No, since the general expression for momentum is related via:

    [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

    Nope.

    Conversvation of total energy and conservation of momentum/impulse is not the same thing.


    They follow the nuclear notion, as pam pointed out.
     
  14. Apr 15, 2008 #13
    E is energy, so what is the difference?
     
  15. Apr 15, 2008 #14

    malawi_glenn

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    i) momentum is a vector, energy is a scalar.

    ii) you have massive particles on the Left Hand Side (LHS) of the reaction.
     
  16. Apr 15, 2008 #15
    So conservation of energy is for the gamma ray, and conservation of momentum for the particles, right?
     
  17. Apr 15, 2008 #16

    malawi_glenn

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    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
     
  18. Apr 15, 2008 #17
    And what is conservation of impulse, said with simpler words? Maybe some analogy?
     
  19. Apr 15, 2008 #18

    malawi_glenn

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    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.
     
  20. Apr 15, 2008 #19
    but what is the impulse of the mass objects? Massless objects like gamma ray, the impulse is that line (which is not straight)
     
  21. Apr 15, 2008 #20

    malawi_glenn

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    whar are you talking about? Impuse is that line (which is not straight)?! :confused:

    Instead of talking about impulse, lets just use the word momentum in the future, to avoid misunderstanding ;)

    The momentum of massive obejcts is (if non relativistic) p = mv
    if relativistic particles, use relativistic kinematics.
     
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