# Decay of an unstable particle: Compute the speeds

An unstable particle at rest breaks up into two fragments of unequal mass. The mass of the lighter fragment is 2.81E-28 kg, and that of the heavier fragment is 1.31E-27 kg. If the lighter fragment has a speed of 0.803c after the breakup, what is the speed of the heavier fragment? Give answer as a fraction of the speed of light.

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#### Solution Preview

You can solve this problem using conservation of momentum. The unstable particle was at rest and thus had a momentum of zero. This means that after the break up the total momentum of the two fragments has to be zero. So, if the lighter fragment has a mass of m1 and is moving at a speed of v1, and the heavier fragment with a mass of m2 has a speed of v2, then that ...

#### Solution Summary

We show how, using conservation of momentum, one can compute the speeds of the decay products.