Problem: An unstable particle at rest breaks up into two fragments of unequal mass. The mass of the lighter fragment is equal to 2.10x10-28 kg and that of the heavier fragment is 1.67 x 10-27 kg. If the lighter fragment has a speed of 0.893c after the breakup,
Part I: What is the speed of the heavier fragment?
I know the correct answer for above problem but explain how to get this answer! Thanks.