plik


ÿþ49. Energy and momentum are conserved. We assume the residual thorium nucleus is in its ground state. Let K± be the kinetic energy of the alpha particle and KTh be the kinetic energy of the thorium nucleus. Then, Q = K± + KTh. We assume the uranium nucleus is initially at rest. Then, conservation of momentum yields 0 = p± + pTh, where p± is the momentum of the alpha particle and pTh is the momentum of the thorium nucleus. Both particles travel slowly enough that the classical relationship between momentum and energy can be used. Thus KTh = p2 /2mTh, where mTh is the mass of the Th thorium nucleus. We substitute pTh = -p± and use K± = p2 /2m± to obtain KTh = (m±/mTh)K±. ± Consequently, m± m± 4.00 u Q = K± + K± = 1+ K± = 1+ (4.196 MeV) = 4.27 MeV . mTh mTh 234 u

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