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Relativistic Momentum of photon

  • #1

Homework Statement


How much work is required to accelerate a proton from rest up to a speed of 0.999c?
What would be the momentum of this proton?

Homework Equations


p=γmv

The Attempt at a Solution


I got part A, which was the momentum. I found that to be 20.1 GeV. Now for part B I have to find the momentum in units of GeV/c. I'm down to my last attempt on Mastering Physics. First, I tried going the simplest route and divided the 20.1 GeV from part A by c to get 6.7e-8 and that was wrong. I also tried using p=γmv to find an answer of 1.12e-5 J, which I converted to 7.01e-8 GeV, then divided that by c and got an incorrect figure of 2.34e-16.

Any help would be greatly appreciated.
 

Answers and Replies

  • #2
PeroK
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Homework Statement


How much work is required to accelerate a proton from rest up to a speed of 0.999c?
What would be the momentum of this proton?

Homework Equations


p=γmv

The Attempt at a Solution


I got part A, which was the momentum. I found that to be 20.1 GeV. Now for part B I have to find the momentum in units of GeV/c. I'm down to my last attempt on Mastering Physics. First, I tried going the simplest route and divided the 20.1 GeV from part A by c to get 6.7e-8 and that was wrong. I also tried using p=γmv to find an answer of 1.12e-5 J, which I converted to 7.01e-8 GeV, then divided that by c and got an incorrect figure of 2.34e-16.

Any help would be greatly appreciated.
Can you find a formula that relates the energy to the momentum of a particle?
 
  • #3
Would that be E^2=p^2c^2 + m^2c^4? If that is the correct equation, I came up with an answer of 1.35e-6.
 
  • #4
PeroK
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Would that be E^2=p^2c^2 + m^2c^4? If that is the correct equation, I came up with an answer of 1.35e-6.
You could use that one. But, what about using ##E = \gamma mc^2## and ##p = \gamma mv##?
 
  • #5
Honestly, I'm not quite sure what I would do with those equations. I saw that a guy on Yahoo Answers did K/c=mvγ and that led me to an answer of 7.01e-8. I'm not confident that that is right though. Any idea if my answer of 1.35e-6 from my previous post is right?
 
  • #6
PeroK
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Honestly, I'm not quite sure what I would do with those equations. I saw that a guy on Yahoo Answers did K/c=mvγ and that led me to an answer of 7.01e-8. I'm not confident that that is right though. Any idea if my answer of 1.35e-6 from my previous post is right?
I think your problem is probably understanding ##eV## units. The mass of a particle is normally given in ##MeV/c^2##. For example, the proton mass is about ##938 MeV/c^2##.

The energy of a particle is, therefore, ##E = \gamma m## where ##E## is in ##MeV## and ##m## is the mass in ##MeV/c^2##.

Momentum is given in units of ##MeV/c## and we have ##p = \gamma mv/c## in these units.

In this case ##v/c = 0.999 \approx 1## so we have ##p \approx \gamma m = E##.

These units take a bit of time and practice to get used to, but you can see how using them can simplify the calculations.
 
  • #7
Yes, thank you it makes sense now.
 

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