(a)[2]
Fill in the missing nuclei in this decay equation. $$^{215}_{84}\text{P} \rightarrow {}^{\phantom{0}}_{\phantom{0}}\text{Q} + {}^{\phantom{0}}_{\phantom{0}}\alpha$$
(b(i))[2]
State the principle of conservation of momentum.
(b(ii))[2]
Nucleus P has a speed of $3.2 \times 10^5\,\text{m s}^{-1}$ before the decay. After the decay, nucleus Q is at rest. Determine the speed of the alpha particle after the decay.