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Rest energy and Kinetic Energy of a Photon - velocity?

  • Thread starter max8404
  • Start date
11
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1. 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.


2. Homework Equations

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

3. 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!
 

tiny-tim

Science Advisor
Homework Helper
25,790
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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

2. 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:
 
11
0
Re: congratulations!

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!
 

tiny-tim

Science Advisor
Homework Helper
25,790
249
E is the total energy (or relativistic energy), mc2/√(1 - v2/c2) :smile:
 
11
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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! :)
 

Redbelly98

Staff Emeritus
Science Advisor
Homework Helper
12,038
127
I have a big problem with a photon's velocity being less than c.
 

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