travelling with velocity 
(which may be close to the velocity of light)
of: a particle of mass travelling with velocity (which may be close to the velocity of light)
is: according to Einstein's special theory of relativity,
where 
is the speed of light in a vacuum. [P2.5]
agrees: with the definition of linear momentum in Newtonian mechanics 
for low speeds (when 
), but differs markedly at high speeds.
ensures: that the principle of conservation of momentum is valid at all speeds (up to ). [P2.5]
Copyright 1997, The Open University