physics
posted by Nat .
What is the momentum of a proton travelling at v = 0.95c in the xdirection
in the reference frame?

Photons all travel at velocity c in a vacuum, in any reference frame. The momentum depends upon the frequency. This question has no answer.

will I be able to calculate the momentum if I know the mass of the proton?

Is the question assuming the proton is travelling relativistically or not?

p = gamma m v
gamma = 1/sqrt[1(v/c)^2] 
The mass of a photon is zero; yet it has momentum. The momentum (as well as th energy) depends upon the frequency, which you have not provided.

Nat, use what Count Iblis provided.
PS: drwls, the question asks about the momentum of a proton, not a photon.