The relativistic total energy of a particle is given by: [tex]E^2= (pc)^2+(m_0 c^2)^2 [/tex] where p is the particle momentum c is the speed of light [tex]m_0[/tex] is the rest mass of the particle
If we re-arrange the equation, we find [tex]p= \frac{1}{c} \sqrt{(E^2-(m_0c^2)^2} [/tex] and by using [tex]c=3 \cdot 10^8 m/s[/tex] [tex]m_0 = 1.67 \cdot 10^{-27} kg[/tex] (proton mass)
we find the momentum of the proton: [tex]p= \frac{1}{3\cdot 10^8 m/s} \sqrt{(3.0 \cdot 10^{-10}J)^2-(1.67\cdot 10^{-27}kg (3\cdot 10^8 m/s)^2)^2} =[/tex] [tex]=8.65 \cdot 10^{-19} kg m/s[/tex]
