text: calculate the momentum of an electron moving with a speed of 0.15c?   the above answer is not quite right.  in relativistic mechanics, momentum is defined as:    p = g * m * v  where  g = 1/sqrt(1-(v/c)^2), v is the velocity of the object in question (relative to some observer),  c is the speed of light,  and m is the rest mass of the moving object.   (strictly speaking, both p and v are vector quantities.)  for |v|<<c, g is approximately equal to 1, and the relativistic momentum approaches m*v, the classical newtonian value of the momentum.    in the case at hand, g = 1/sqrt(1-0.15^2) = 1.01144, so the relativistic momentum of the electron is 1.01144 times the classical value (i.e., 1.144% higher than just m*v).