Find the group and phase velocities of 10-MeV protons and 10-MeV electrons (see Problem...

Find the group and phase velocities of 10-MeV protons and 10-MeV electrons (see Problem 28).

Problem 28

A wave packet describes a particle having momentum p. Starting with the relativistic relationship ,show that the group velocity is c and the phase velocity is c/ β (where β = v/c). How can the phase velocity physically be greater than c ?