A nucleus of uranium decays at rest into nuclei of thorium and helium. Then

If p→Th and p→He  are the momenta of thorium and helium nuclei respectively, then according to law of conservation of linear momentum0=p→Th+p→He or p→Th=-p→He-ve sign shows that both are moving in opposite directions. But in magnitudepTh=pHeIf mTh and mHe are the masses of thorium and helium nuclei respectively, then Kinetic energy of thorium nucleus is KTh=pTh22mTh and that of helium nucleus is KHe=pHe22mHe∴  KThKHe=pThpHe2mHemTh But pTh =pHe and mHeKThThus the helium nucleus has more kinetic energy than the thorium nucleus.