Rest energy and Kinetic Energy of a Photon - velocity?

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max8404
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Homework Statement



Thanks for everyone that helped me with the physics questions! I had my physics 2 final today and got a 94! There was a question that I could not figure out for a long time. It goes like this.

What would the velocity of the photon be if the Kinetic energy of the photon equals the rest energy of the photon? Express v in terms of c and anything else. Luckily, the question was a multiple choice, and the correct answer (which I guessed) was sqrt(3)/2 x c

I still cannot figure out why this is true.


Homework Equations



K=1/2mv^2 --> I assume this is the right equation to use...
Er=mc^2

The Attempt at a Solution



Equaling the two would give you:

1/2mv^2=mc^2

v^2=2c^2
v=sqrt(2) x c

This is obviously not true since you can't go faster than the speed of light. Can someone explain to me how the answer is sqrt(3)/2 x c and why I am getting this thing wrong? Am I using the right Kinetic Energy equation??

Thanks!
 

Answers and Replies

  • #2
tiny-tim
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congratulations!

Hi max8404! Many congratulations! :biggrin:
What would the velocity of the photon be if the Kinetic energy of the photon equals the rest energy of the photon? Express v in terms of c and anything else. Luckily, the question was a multiple choice, and the correct answer (which I guessed) was sqrt(3)/2 x c

Homework Equations



K=1/2mv^2 --> I assume this is the right equation to use...
… why I am getting this thing wrong? Am I using the right Kinetic Energy equation??

oooh, I wish examiners wouldn't use "kinetic energy" in relativity :cry:

they just mean total energy minus rest energy :rolleyes:

so 2m = m/√(1 - v2/c2), so 1 - v2/c2 = 1/4 :wink:
 
  • #3
max8404
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Hi max8404! Many congratulations! :biggrin:


oooh, I wish examiners wouldn't use "kinetic energy" in relativity :cry:

they just mean total energy minus rest energy :rolleyes:

so 2m = m/√(1 - v2/c2), so 1 - v2/c2 = 1/4 :wink:

Hey Tiny Tim, thanks for your response, but I doon't quite get it. I understand the equation,
E=KE+mc^2, but in this case, what would E be? Or am I once again looking at it the wrong way?

Thanks!
 
  • #4
tiny-tim
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E is the total energy (or relativistic energy), mc2/√(1 - v2/c2) :smile:
 
  • #5
max8404
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E is the total energy (or relativistic energy), mc2/√(1 - v2/c2) :smile:

duh. thanks a lot! I got it now! :)
 
  • #6
Redbelly98
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I have a big problem with a photon's velocity being less than c.
 

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