The relativistic formula for the momentum of a particle of mass m and moving at speed v is
[tex]p= \frac{mv}{ \sqrt{1- \frac{v^2}{c^2} } } [/tex]
where c is the speed of light.
In our problem, the speed in terms of c is [tex]v=0.805 c[/tex], and the mass (still written in terms of c) is [tex]m=938 Mev/c^2[/tex]. Substituting these values in the previous formula, we can find the value of the momentum of the proton:
[tex]p= \frac{(938 MeV/c^2)(0.805 c)}{ \sqrt{1-( \frac{0.899c}{c} )^2} } =1273 MeV/c[/tex]
