7 MeV a-particle before it
comes momentarily to rest and reverses its direction Solution The key idea here is that throughout the scattering process,
the total mechanical energy of the system consisting of an a-particle
and a gold nucleus is conserved The system’s initial mechanical
energy is Ei, before the particle and nucleus interact, and it is equal
to its mechanical energy Ef when the a-particle momentarily stops The initial energy Ei is just the kinetic energy K of the incoming
a- particle