A uranium nucleus (mass 238 u), initially at rest, undergoes radioactive decay. After an alpha particle (mass

Question:

A uranium nucleus (mass 238 u), initially at rest, undergoes radioactive decay. After an alpha particle (mass 4.0 u) is emitted, the remaining nucleus is thorium (mass 234 u). If the alpha particle is moving at 0.050 times the speed of light, what is the recoil speed of the thorium nucleus? ("u" is a unit of mass; it is not necessary to convert it to kg.)