# Homework Help: Calculate the momentum

1. Sep 22, 2008

### 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.

2. Sep 22, 2008

### LowlyPion

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?

3. Sep 22, 2008

### cepheid

Staff Emeritus
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.

4. Sep 22, 2008

### melissajohn

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

5. Sep 22, 2008

### LowlyPion

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.

6. Sep 22, 2008

### 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!