Problem: An observer at rest in a laboratory sees particles emitted from a source. The first bunch of particles is emitted with speed v=0.92c; the second bunch emitted 10^-3 s later with v=0.99c. How long, according to the observer, will it be until the second bunch catches up with the first? I'm not sure how straight forward this problem is. Using "classical" physics I get T=1.3*10^-2 s I am not sure what to account for in this problem....relativistic relative velocity maybe?