# Homework Help: Momentum unit

1. Sep 24, 2014

### Safinaz

Hello,
I were wondering how to say the unit of the momentum by words, for example if the momentum equals 15 GeV/c , is it right to say:
The momentum is 15 giga electron volts divided by the light velocity ?
will this be clear for an audince, since ' c ' here can equals 1 at natural units .. so can one say:
15 giga electron volts only .

Bests,
S.

2. Sep 24, 2014

### Orodruin

Staff Emeritus
Particle physicists would typically work in units with c = 1 and use eV (with prefixes) as units of both energy, mass, and momentum. I do not think that anyone will misunderstand you if you just say "giga electron volts" - there are not any other constants with unit length/time that could reasonably be set to 1 and the conversion to "real" momentum units should be clear.

3. Sep 24, 2014

### Safinaz

That make me ask, is there any physical meaning, why the natural unit are there, where constants like c and the Plank constant
equal unity ..
For momentum or mass, is that due to the experimentalist can measure these quantities as energies ? but what about the Plank constant;
E = h $\nu$ .. is it means the frequency of an atomic oscillator equavlint to its energy , but is still units problem here ? since E has GeV and
$\nu$ has sec^-1 ..

4. Sep 24, 2014

### Staff: Mentor

In particle physics, it is just "15 GeV" (the GeV usually pronounced as individual letters). You might see "15 GeV/c" in publications, but it would sound really odd to hear that in a talk/discussion.

You can set those constants to one because they are "just" unit conversions. Their numerical value changes depending on your unit system (as an example, if you express the speed of light in miles per hour, you get a different value), so you can choose a unit system where they are 1.

5. Sep 24, 2014

### Orodruin

Staff Emeritus
Just to add to this, some people pronounce it as if it was an actual word with the "G" pronounced like the g in "germanium".

6. Sep 25, 2014

### Safinaz

A last questin, if Plank constant in ;
E = h ν , equals 1 , doesn't this make mismatch between units at both sides of the equation ?

7. Sep 25, 2014

### Orodruin

Staff Emeritus
It is the same as with putting c to one but with $\hbar$ instead. With $\hbar = 1$ and c=1, length and time both have the same unit as 1/energy.

In fact, people who do kaluza klein theories often use GeV as a unit for 1/R, where R is the size of their extra dimensions.

8. Sep 25, 2014

Thanks ..