# law of impulse

by Physicsissuef
Tags: impulse
P: 909
Annihilation (connecting positron and electrons) gives us 2 gamma rays. I.e
$$^{0}_{-1}e+ ^{0}_{+1}e \rightarrow 2 \gamma$$
In my textbook says:
 Quote by mytextbook The pair positron-electron is processing according to the laws of preservation of the energy and the impulse... Impulse of the gamma quant is equal to the both particles, same as the nucleus (which is in interaction with), but, the summary impulse is same before and after the process... That is confirmed in The Wilson's chamber which is inside of magnetic and electric field. In that field the electron and the positron like opposite charged particles are going into opposite directions.
Now my question, what is law of preservation of impulse??
Why $$e$$ is written like $$^{0}_{-1}e$$ instead of $$e^-^1$$ or something??
 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.
P: 909
 Quote by pam 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.
Yes, I mean conservation of momentum... What is that?

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

$$\textbf{p}=m\textbf{v}$$

where m is the mass and v the velocity of the particle.
For photons:

$$|p|=\frac{E}{c}$$

where E is the enery and c the speed of light (in vacuum).
 P: 909 in this case, it imply for the gamma ray? So $$|p|=\frac{E}{c}$$ is the one that we look for, right?
 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.
 P: 909 so mv=$\frac{E}{c}$, right?
 P: 909 and why p is in long brackets i.e |p| ?
 P: 909 is it same with conservation of energY?
Mentor
P: 11,038
 Quote by Physicsissuef and why p is in long brackets i.e |p| ?
Momentum is a vector. It has both magnitude and direction. The vertical bars indicate that we are talking about the magnitude only.
 P: 909 conservation of energy and conservation of impulse are same in this casE?
HW Helper
P: 4,739
 Quote by Physicsissuef so mv=$\frac{E}{c}$, right?
No, since the general expression for momentum is related via:

$$E^2 = p^2c^2 + m^2c^4$$

So for massless particles (as the photon):

$$P = E/c$$

There is no way that you can get mv = E/c

 Quote by Physicsissuef conservation of energy and conservation of impulse are same in this casE?
Nope.

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

 Quote by Physicsissuef Why $$e$$ is written like $$^{0}_{-1}e$$ instead of $$e^-^1$$ or something??
They follow the nuclear notion, as pam pointed out.
 P: 909 E is energy, so what is the difference?
 Sci Advisor HW Helper P: 4,739 i) momentum is a vector, energy is a scalar. ii) you have massive particles on the Left Hand Side (LHS) of the reaction.
 P: 909 So conservation of energy is for the gamma ray, and conservation of momentum for the particles, right?
 Sci Advisor HW Helper P: 4,739 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
 P: 909 And what is conservation of impulse, said with simpler words? Maybe some analogy?
 Sci Advisor HW Helper P: 4,739 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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