A radioactive nucleus of uranium-235 (mU = 235 u) decays spontaneously to thorium-231
by emitting an alpha particle (nucleus of a helium atom, symbolised α). The α particle
(mα = 4.00 u) has kinetic energy 4.60 MeV. What is the kinetic energy of the thorium-231
nucleus (mT h = 231 u)?