The momentum p of a moving particle is the product between its mass, m, and tis velocity, v:
[tex]p=mv[/tex]
In our problem, we know [tex]p=10^{-16}~Kg \cdot m/s[/tex] and [tex]v=0.87c=0.87\cdot 3\cdot10^8~m/s=2.61\cdot 10^8~m/s[/tex], and using the relationship mentioned above, we can find the mass m of the particle:
[tex]m= \frac{p}{v} =3.8\cdot10^{-25}~Kg[/tex]
