Homework Help: Speed of a Relativistic Particle?

1. Dec 2, 2012

bmb2009

1. The problem statement, all variables and given/known data

Particle track detectors are used to measure the speed of particles if the lifetime of the particle is known. Particle X has a lifetime of 256.2 ps. These particles are created in an experiment inside the detector by a given reaction. The particles leave 21.8 cm long tracks on average before they decay into other particles not observable by the detector.
What is the average speed of the particles in terms of the speed of light?

2. Relevant equations

Not sure?

3. The attempt at a solution
While this is a question of relativistic speed of the particle... I'm not sure what the average track length relates to the lifetime of the particle? Can anyone point me in the direction of the relevant equations? Thanks

2. Dec 5, 2012

Ibix

The particles come into existence inside the detector. They travel some distance in some time, then decay and disappear. If this were non-relativistic, you could just do d=vt, sub in the lifetime as t and the track length as d. But it's relativistic - what do you do?

PS: You might want to look up muons in relation to tests of special relativity.