Photon Momentum Calculation for 25 x 105 erg Energy

[melissajohn]

how do you calculate the momentum if the energy of photon is 25 x 105 erg?

i know p=mass*velocity
but i am not sure how to do this problem.

is the SI units of photons... erg? i just want to make sure.

 

[LowlyPion]

how do you calculate the momentum if the energy of photon is 25 x 105 erg?

i know p=mass*velocity
but i am not sure how to do this problem.

is the SI units of photons... erg? i just want to make sure.

What are ergs? Units of energy? units of mass distance and distance/sec²

And rearranging what is that mass * distance²/s²?

And Einstein's famous equation is E= ... what? mass times what?

And what units is that?

 

[cepheid]

The SI unit of energy is the joule. An erg is a unit of energy in the cgs system where you use centimetres, grams, and seconds (hence cgs) as the fundamental units of length, mass and time, INSTEAD of metres, kilograms, and seconds.

Photons don't have mass, but they do carry momentum. This may seem strange, but it's because a photon's momentum is NOT given by p = mv. This equation is NOT valid for massless particles such as photons. You need to find out the relationship between energy and momentum for a photon.

 

[melissajohn]

ergs - yes units of energy.
what do you mean rearranging the equation?
Einstein equation is e=mc^2?

 

[LowlyPion]

ergs - yes units of energy.
what do you mean rearranging the equation?
Einstein equation is e=mc^2?

I was merely pointing out that energy was in the same units you would find in the E=mc²

But that's not all because from that you can also come up with an expression for the momentum. While a photon has no mass it can be expressed as E = p*c or p=E/c.

Knowing the units of ergs and the speed of light in the same units then you might be able to determine the answer.

 

[melissajohn]

ok so...
if i want to calculate the momentum if the energy of photon is 25 x 105 erg

I would get...

p=E/c
(25*10^5 erg)/(3*10^8 m/s)= 8.3*10^-3 erg*m/s

is that correct?

thanks!