Calculating Photon Energy with Proton Momentum | Special Relativity Homework

[lockedup]

Homework Statement

What is the energy of a photon whose momentum is the same as that of a proton with a kinetic energy of 10 MeV?

Homework Equations

K = mc^{2}(\gamma-1)
p = \gammamv
E = pc

The Attempt at a Solution

I figured I would go at it like this. I used the first equation listed to obtain a value for gamma and v. I put these into the second equation along with the mass of a proton. Then, I would put that value into the third equation. Is that even close?

 

[Lok]

Seems ok if I understood it right. Your equations might help more.

 

[torquil]

As far as I can understand, the procedure you describe is correct. As long as you have done the calculations properly, you should end up with the correct answer.

Torquil

 

[lockedup]

Ok, so I have \gamma = 1 + 1.18 \times 10^{-19} So, um, how am I supposed to square that (so I can find beta)? My calculator won't go that far... :*)

 

[dacruick]

use an online calculator

 

[dacruick]

or any computer program will do it

 

[dacruick]

or you can just square the 1.18 and then double 19

 

[lockedup]

use an online calculator

They all give me 1

 

[dacruick]

the answer is 1.3924e-38

 

[dacruick]

so for next time this is what you will do. you should square 1.18. then you should double the exponent. and voila

 

[lockedup]

so for next time this is what you will do. you should square 1.18. then you should double the exponent. and voila

You're misreading the problem. If it was just 1.18e-19, I wouldn't have a problem. I need to square 1+1.18e-19. BTW, I found a calculator that will do it. Google can be tricksy sometimes...

 

[jdwood983]

if \gamma=1+1.18\times10^{-19} then you should just approximate \gamma\simeq1.

Also, use www.wolframalpha.com It's like having Mathematica available to you with 0 cost

 

[vela]

Your value for \gamma is way too small. The proton's kinetic energy is about 1% of the rest energy, so \gamma should be about 1.01.

 

[Altabeh]

Homework Statement

What is the energy of a photon whose momentum is the same as that of a proton with a kinetic energy of 10 MeV?

Homework Equations

K = mc^{2}(\gamma-1)
p = \gammamv
E = pc

The Attempt at a Solution

I figured I would go at it like this. I used the first equation listed to obtain a value for gamma and v. I put these into the second equation along with the mass of a proton. Then, I would put that value into the third equation. Is that even close?

The speed of light = 299,792,458 m/s;
Proton mass: 1.67262158*10^{-27} kg;
K= 1.602176487*10^{−15} J.

So with a simple mind-based calculation you at least can get that the Lorentz factor is around 1. The following shows its exact value up to 9 decimal digits:

\gamma=1.000010658.

AB

 

[chrispb]

This is such a complicated way to do this problem. The energy of a proton is its kinetic energy plus its mass, and that squared is p^2 + m^2 (with c=1). Find p. E(photon) = p. Done.