A stationary radium atom decays, emiting an alpha particle. Why is the recoil speed of the nucleus small compared to the alpha particle?
Initially the momentum is zero.
Due to conservation of momentum, the alpha particle and radium nucleus must gain equal but opposite momentum.
The mass of the radium nucleus is greater than the mass of the alpha particle.
Therefore, the alpha particle has a much greater speed after emission than the radium nucleus. (p = mv)
