An atomic nucleus of radon initially moving at 515 m/s emits an alpha particle in the direction of its velocity, and the new nucleus slows to 417 m/s. If the alpha particle has a mass of 4u, and the original nucleus of radon had a mass of 222u, what speed does the alpha particle have when it is emitted?

Answer 1

Use conservation of momentum

222*515 = 417*(222-4) + 4*valpha

where valpha is the speed of the alpha particle