(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A K-meson at rest decays into two pi-mesons, and each pi meson has a speed of 0.85c.

If a K-meson travelling at a speed of 0.9c decays, what is the greatest speed the one of the pi-mesons can have?

2. Relevant equations

[tex]u\prime = \frac{u-v}{1-\frac{v}{c^2}u}[/tex]

3. The attempt at a solution

After plugging in v=0.9c and u=0.85c I get 3 different answers (3 different derivations of the above equation)

Initially I got 1.11c which is clearly wrong, I think that answer came from just not cancelling properly and putting things in my calculator wrong.

Next I got a value of -0.213c from the above equation that I just copied from my notes.

Finally I got a value -0.213c from a similar equation to that above, but with all the signs on the RHS switched, when I tried to derive the equation. (I didn't actually know that this was the same answer - I've only just worked out the fractional value while typing this!). I am however confused, as surely a complete change of sign throughout the equation would change the sign of the answer. So why have I got the same answer?

Any help at all would be gratefully received - my main problem with Special Relativity is deciding which equation to use for the data given.

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# Special Relativity - addition of velocities

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