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## Homework Statement

The pion has an average lifetime of 26.0ns when at rest. For it to travel 10.0m, how fast must it move?

## Homework Equations

Lorentz velocity transformation?

## The Attempt at a Solution

I'm very lost... am I supposed to use u'

_{x}= (u

_{x}-v)/(1-vu

_{x}/c

^{2})? I thought I was following the lecture pretty well, but now that I'm trying to solve a problem, I have no idea what to do.

After some more poking around, I used Δt=γΔt'. Now I have one equation and two unknowns (v and Δt). I know I am supposed to use a velocity equation to find v now, but I'm still not sure which eqn to use, and how.

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