Calculating the "proper mean lifetime" of pions

matt_crouch

1. The problem statement, all variables and given/known data
The mean lifetime of pions at rest is to=2.6x10^-8. if a beam of pions has a speed v=0.85c

a)what would their lifetime be in the laboratory?
b)how far would they travel before they decay?
c)what would your answer to b) if you neglect time dilation
d)what would the "proper mean lifetime" be?



2. Relevant equations



3. The attempt at a solution

i think done a-c right but dont really know how to do d)

my answers

a)4.94x10^-8
b)12.5 m
c)6.63m

im not sure how to calculate the proper mean lifetime can anyone help?