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HelpPlease27

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## Homework Statement

After being produced in a collision between elementary particles, a positive pion (π+) must travel down a 1.00 km -long tube to reach an experimental area. A π+ particle has an average lifetime (measured in its rest frame) of 2.60×10−8s; the π+ we are considering has this lifetime.

How fast must the π+ travel if it is not to decay before it reaches the end of the tube? (Since u will be very close to c, write u=(1−Δ)c and give your answer in terms of Δ rather than u.)

The π+ has a rest energy of 139.6 MeV. What is the total energy of the π+ at the speed calculated in part A?

## Homework Equations

Δ t = Δt0 / sqrt(1-u^2/c^2)

E^2 = (mc^2)^2 + (pc)^2

## The Attempt at a Solution

I got the correct answer for speed, the first part of the question. It's the second part I can't get to work. I used the total energy equation and my speed, which worked out to give me E = 197.4 MeV but this wasn't right. I'm not sure where I'm going wrong?