i dont understand when to use substitution as used in the answer to this question:(adsbygoogle = window.adsbygoogle || []).push({});

how fast must a pion be moving, on average, to travel 10m before it decays? average lifetime is 2.6*10^-8.

i know the answer is D=V( to/ sqroot 1-v^2/c^2) but i dont understand why and how to know when to put the time expansion equation into the D=vt equation.

When would u ever do this again, and why isnt the pion's lifetime= t, b/c its at rest.

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# Special relativity and substitution

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