The rest mass of the muon is 207 times the mass of the electron:
[tex]m=207 m_e = 207 \cdot 9.1 \cdot 10^{-31} kg = 1.88 \cdot 10^{-28}kg[/tex]
The relativistic momentum of a particle is given by
[tex]p= \gamma m v[/tex]
where
m is the particle's mass
v is its velocity
[tex]\gamma = \frac{1}{ \sqrt{1- \frac{v^2}{c^2} } } [/tex] is the relativistic factor, with c being the speed of light.
The muon is traveling at speed [tex]v=0.981 c[/tex], so the relativistic factor is
[tex]\gamma = \frac{1}{ \sqrt{1- (\frac{0.981 c}{c})^2 } } =5.154[/tex]
So the momentum of the muon is
[tex]p= \gamma m v = (5.154)(1.88 \cdot 10^{-28}kg)(0.981 \cdot (3 \cdot 10^8 m/s))=[/tex]
[tex]=2.85 \cdot 10^{-19} kg m/s[/tex]
