# B Momentum formulae

1. Apr 14, 2017

### rahaverhma

I know that P=m*v but I saw that
Momentum (p) =E/c how can we say this
We need to have inertia to call the momentum
of entity. Plzz solve this dilemma

2. Apr 14, 2017

### Staff: Mentor

Conservation of momentum requires that light have momentum. This can be seen in things like the radiation pressure on an object in space. If we didn't assign momentum to light then there would be a violation of the conservation of momentum law.

3. Apr 16, 2017

### SergioPL

When you say P=m*v you mean that the momentum P is mass multiplied by velocity, both them are vectors and m is the inertial mass (different than the rest mass in relativistic mechanics).

On the other hand |p| = E/c is a equation that only applies to photons, photons have zero rest mass but the inertial mass is m = E/c^2 and since v=c this leads to

|p| = m * c = E / c

4. Apr 17, 2017

### vanhees71

Not again please! It is really good advice not to use any other notion of mass in relativity than invariant mass, and the invariant mass of photons is 0 and nothing else. $E/c^2$ is just the energy divided by $c^2$. In natural units you have $c=1$ anyway, because in relativity there's no reason to use different units for space and time or energies and momenta.