First slide
Energy released in nuclear reactions -Q-value

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


 If pTh and pHe  are the momenta of thorium and helium nuclei respectively, then according to law of conservation of linear momentum

0=pTh+pHe or pTh=-pHe

-ve sign shows that both are moving in opposite directions. But in magnitude


If 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 



 But pTh =pHe and mHe<mThKTh<KHe or KHe>KTh

Thus the helium nucleus has more kinetic energy than the thorium nucleus.

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